Engineering & Inventing for Kids (Grades 2-5)

A party for a bridge

On 24 May 1883, schools and shops in New York closed for the day. Crowds filled the streets by the river. A new bridge was opening, and the whole city wanted to see it.

The Brooklyn Bridge crosses the East River in New York. Its main span, the part that hangs between its two tall towers, is 1,595 and a half feet long. That is more than 500 big grown-up strides. No bridge in the world had a span that long.

The bridge took 14 years to build. It needed hundreds of workers, one very determined family, and a great deal of careful thinking. That careful thinking has a name. It is called engineering.

The family who built it

A man called John A. Roebling drew the plans for the bridge. Then, in 1869, before the building began, he died. His son, Washington Roebling, took charge.

The bridge needed two huge towers standing in the river. To build them, workers went down inside giant boxes called caissons (say it: KAY-sons). A caisson is like an enormous box with no bottom, pushed down into the mud under the water. Air is pumped in to keep the water out, so workers can dig underneath.

The air inside was squeezed tight. Long hours in that air made many workers very ill. Doctors call the illness the bends. In 1872, Washington Roebling was carried up from a caisson, unconscious. He lived, but he was too ill to go to the bridge again. He could only watch the work from the window of his sickroom.

So his wife, Emily Warren Roebling, stepped in. She studied hard. She learned how strong different materials are, how forces push and pull on a bridge, and how bridge cables are made.

Every day she went to the bridge. She passed on her husband's instructions to the workers, and she answered their questions. The American Society of Civil Engineers says the bridge might not have been finished without her careful checking and organizing. On opening day, Emily rode across the new bridge with the President of the United States, Chester Arthur.

Building the bridge was dangerous. About 20 workers died while it was being built. Engineers today plan hard to keep builders safe. They learned from stories like this one.

Key idea: A big project needs people who plan, test, check and keep going when things go wrong. Emily Roebling did all four.

So what is an engineer?

An engineer is a person who designs and builds things that solve problems for people. To design something means to plan how it will look and how it will work before you make it.

To an engineer, a problem is anything people want to change or make better. The people of Brooklyn and New York wanted to walk, ride and drive across a wide river. That was their problem. The bridge was the answer.

The science standards used in many American schools say the same thing in grown-up words. A situation that people want to change can be treated as a problem to be solved through engineering. Notice the word people. Engineering always starts with people and what they need.

Here is how a problem turns into an engineering job.

1. People notice something they want to change. We want to get across the river.
2. Engineers ask questions. How wide is the river? How heavy are the carts? How strong is the wind?
3. They come up with ideas, and they choose one and plan it carefully.
4. They build it, test it, and make it better.

You will learn these steps properly in the next lesson. They have names: ask, imagine, plan, create and improve.

The point: Engineering means solving a real problem for real people by designing, building and testing.

The made world and the natural world

Look out of a window. Some things you see were made by nature: trees, clouds, rain, rocks and birds. That is the natural world.

Other things were made by people: the window glass, the road, the street light, the car and the fence. That is the made world. Almost everything in the made world was designed by somebody first.

Some things are a mix. A wooden table is made from a tree, and trees are natural. But somebody chose the shape of the table and built it. So the table belongs to the made world.

For the grown-up helping: Children often call a thing natural if it is made of a natural material, such as wood or wool. The useful question is: did a person decide its shape? If yes, it is part of the made world.

Many kinds of engineers

There are many kinds of engineers. Each kind works on a different part of the made world. The U.S. Bureau of Labor Statistics keeps track of jobs in America, and it lists more than a dozen kinds of engineers. Here are some of them.

Emily and Washington Roebling did the work of civil engineers. In this course you will try the work of several kinds. You will build bridges and towers like a civil engineer. You will build machines like a mechanical engineer, planes and parachutes like an aerospace engineer, and a circuit like an electrical engineer.

Key idea: Engineers are not all the same. They share one way of working, but they use it on very different problems.

What engineers do all day

Engineers do not spend all day building. Much of their work happens before anything is made, and after it is made.

• They ask questions. Who will use this? What must it do? What could go wrong?
• They draw and plan. A drawing lets other people see the idea before it exists.
• They build models. A model is a small or simple version of a design, made to try out an idea. The first working version of a new design is called a prototype.
• They test. A test shows whether the design really works.
• They improve. Almost nothing works perfectly the first time. Engineers fix what failed and test again.
• They work in teams. Emily Roebling could not have built the bridge alone, and neither could her husband.
• They write things down. Notes, numbers and drawings help the team remember what they tried.

You will do every one of these things in this course. Get a notebook just for engineering. Engineers call it an engineering notebook. Every build, every test and every result goes in it, with the date.

Engineers and scientists

Engineers and scientists often work side by side. But they ask different questions.

A scientist asks: how does the world work? Why does ice float? Why is the sky blue?

An engineer asks: how can we use what we know to make something people need? How can we build a boat that floats with a heavy load? How can we light a room at night?

Both of them measure carefully, and both of them test their ideas. The difference is the goal. A scientist wants to understand the world. An engineer wants to build something that works in it.

Try it: natural or made?

Say natural or made for each one. Then say what problem each made thing solves.

1. A pebble.
2. A pair of glasses.
3. A spider's web.
4. A ramp up to a front door.
5. A raincoat.
6. An apple on a tree.

Answers. One: natural. Two: made; glasses help people see clearly. Three: natural; a spider built it, but no person designed it. Four: made; a ramp lets wheelchairs, strollers and wheeled suitcases roll up to the door. Five: made; a raincoat keeps you dry. Six: natural.

Look, but do not take apart

Engineers love to know how things work. That is a good habit. But never open, unscrew or take apart anything that plugs into the wall. Never open anything with a battery inside unless a grown-up says it is safe. Machines can have sharp edges and electrical parts inside.

Common misconceptions

• Mistake: engineers fix engines. The word sounds like engine, but most engineers design things. A mechanic is the person who fixes a car engine. Engineers work on everything from bridges to hospital machines.
• Mistake: a good engineer gets it right the first time. Good engineers expect their first try to have problems. That is why they test.
• Mistake: you have to be a grown-up to do engineering. You can do it right now, at a kitchen table. Later in this course you will meet an inventor who started at fifteen.
• Mistake: engineering is the same as science. They share tools, like measuring and testing. Science tries to explain the world. Engineering tries to build things that work in it.
• Mistake: engineering is a job for men. Emily Warren Roebling helped finish one of the most famous bridges in the world in 1883. Engineers today are women and men from every background.

Looking back

• The Brooklyn Bridge opened on 24 May 1883, after 14 years of work.
• Emily Warren Roebling studied engineering and carried her sick husband's instructions to the workers every day.
• An engineer designs and builds things that solve problems for people.
• The made world is everything people have designed. The natural world is everything nature made.
• There are many kinds of engineers, such as civil, mechanical, electrical and aerospace engineers.
• Engineers ask, plan, build models, test, improve, work in teams and write things down.

Sources

1. American Society of Civil Engineers. (n.d.). Emily Warren Roebling. Notable civil engineers. asce.org
2. American Society of Civil Engineers. (n.d.). Brooklyn Bridge. Historic civil engineering landmarks. asce.org
3. American Society of Civil Engineers. (n.d.). Washington Augustus Roebling. Notable civil engineers. asce.org
4. U.S. Bureau of Labor Statistics. (n.d.). Architecture and engineering occupations. Occupational Outlook Handbook. bls.gov
5. NGSS Lead States. (2013). K-2-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

Five steps on one hand

In January 2016, Cynthia Berger wrote a short article for Engineering is Elementary, a program run by the Museum of Science in Boston. She explained why the program's design process for children has exactly five steps. Elementary school teachers had told the team that five is a magic number for kids. You can count five steps on one hand.

A design process is a set of steps that guides you through solving a problem. Hold up one hand and count the five steps with me.

1. Ask. What is the problem? What must the design do? What are the limits?
2. Imagine. Think of lots of possible answers. Then pick the one that seems best.
3. Plan. Draw the design, label it, and list the materials.
4. Create. Build it, and test it.
5. Improve. Look at what happened in the test, and make the design better.

Then you can go round again. The museum calls the process a cycle, which means a loop that comes back to the start. It is not a straight road that you walk down once. Engineers move back and forth between the steps, and they make a design better by going round the loop again.

Remember: Ask, imagine, plan, create, improve. Then go round again.

For the grown-up helping: The five steps are a simple version made for young children. The museum's own pages say that working engineers often use processes with more steps, and its newer program for schools shows Test as a step of its own between Create and Improve. In the five-step version, testing happens during Create. Either way, nothing gets improved until it has been tested.

The challenge: a zip line for coins

A zip line is a rope stretched from a high place down to a low place. Things hang from it and slide down. Your challenge is to build a carrier that slides down a string zip line and delivers 10 coins to the bottom without dropping a single one.

Here is what you need.

• About 2 meters of string, such as kite string or garden twine
• A chair, and a table leg about 2 meters away from it
• 1 paper cup
• 4 paper clips
• 1 drinking straw
• 1 sheet of paper
• 30 centimeters of tape
• Safety scissors
• 10 coins, all of the same kind
• A ruler or a tape measure

To set up the zip line, tie one end of the string to the back of the chair, about as high as your waist. Tie the other end to the table leg, close to the floor. Pull the string tight before you tie the second knot.

Safety: Keep the string low, below the height of anyone's neck, and take it down when you finish. Never put string around anyone's neck. An opened paper clip has sharp ends, so point them away from your face. Coins are small enough to choke a baby or a toddler, so keep them away from younger brothers and sisters.

Step 1: Ask

Start with questions. Write them in your engineering notebook.

• What must the carrier do?
• How will it hang on the string?
• What might make coins fall out?
• What materials and how much time do I have?

Your answers turn into two lists. Engineers have special names for them.

Criteria (say it: cry-TEER-ee-uh) are the things a design must do to count as a success. A bridge must hold up cars. A raincoat must keep you dry.

Constraints (say it: con-STRAYNTS) are the limits you must work inside. You might have only one roll of tape, only 20 minutes, or only the things in the recycling box.

The science standards for grades 3 to 5 ask students to do exactly this. You define a simple problem, with criteria for success and constraints on materials, time or cost.

Why this matters: If you do not know what success looks like, you cannot tell whether your design worked.

Step 2: Imagine

Now think of as many ideas as you can. Engineers call this brainstorming. In a brainstorm, every idea gets written down, even the odd ones. You choose later.

Here are three ideas for a coin carrier.

1. A bucket. The paper cup hangs from a paper clip hook.
2. A basket. A sheet of paper folded into a box, with a paper handle.
3. A straw rider. A straw slides along the string, with the cup taped underneath it.

Which one should you pick? Check each idea against the criteria. The paper basket might squash under 10 coins. The straw rider has to be threaded onto the string, so you must untie the string every time you change something. The bucket is strong, holds coins well, and hooks on and off in a second. So the bucket is a good first choice.

Choosing is not the same as being sure. You are making your best guess, and the test will tell you whether it was right.

Step 3: Plan

Draw the bucket in your notebook. Label the cup, the clip and the string. Then write the steps for building it.

1. Open one paper clip into an S shape.
2. Push the bottom curl of the S through the rim of the cup, near the top edge.
3. Hang the top curl of the S over the string.
4. Put the 10 coins in the cup.

A plan like this is useful for a second reason. Anyone on your team can read it and build the same thing.

Step 4: Create, and test

Build the bucket from your plan. Then test it. Hook it on at the top of the string, put in the 10 coins, and let go. Do not push. Count the coins that reach the bottom. Do three runs, and write down each one.

Here is how a first build might go. Your numbers will be different, and that is fine.

The bucket failed the criteria. That is not bad news. It is information. You watched it fail, so you know how it failed: the cup tipped.

Step 5: Improve

Why did the cup tip? It hung from one point on one side of its rim. Picture carrying a bucket of water by holding one side of the top edge. It tips, and water spills out.

So change the way it hangs. Push one paper clip through each side of the rim, opposite each other. Join their top ends to a third clip, and hang that third clip on the string. Now the cup hangs straight, like a bucket held by its handle.

Test again, with the same string, the same slope and the same coins. Only the way the cup hangs has changed. The new runs give 10, 10 and 10. Every criterion is met.

Notice that you changed one thing and kept everything else the same. That made the comparison fair. The next lesson is all about fair tests.

Round the loop again: a steeper string

Now change the zip line. A grown-up unties the top end and ties it higher, at the top of the chair back. The string is now steeper. Run the same two-clip bucket.

This time the carrier races down. It slams into the bottom and the cup swings hard. Coins jump out. Over three runs, 7, 6 and 8 coins arrive.

The bucket did not get worse. The problem changed. A steeper string is a new situation, so you go back to Ask. What must the carrier do now? It must survive a sudden stop.

Imagine again: a lid, a soft bumper at the bottom of the string, or a deeper cup. Plan the lid: a circle of paper a little wider than the cup, taped down along one edge so that it opens like a door. Create it and test it. The coins stay in: 10, 10 and 10.

Why this matters: A design that works in one place can fail in another. Engineers test their designs in the conditions where they will really be used.

More than one right answer

Another engineer might have solved the steep string with a cushion of folded paper at the bottom instead of a lid. That could work too. There is usually more than one good solution to a problem. That is why engineers imagine several ideas, build the best ones, and compare them in tests.

For the grown-up helping: Let the first design fail if it is going to. Resist fixing it. A failure your child can explain is the most useful result this lesson can give.

Common misconceptions

• Mistake: you do each step once, in order, and then you are done. The steps make a loop. You go round as many times as you need, and you can go back a step when you learn something new.
• Mistake: the first idea is the best one. The first idea is just the first. Brainstorming several ideas gives you a better chance of finding a good one.
• Mistake: if the test fails, you failed. A failed test tells you exactly what to improve. Engineers expect failures and learn from them.
• Mistake: improving means starting again from nothing. Keep what works and change what failed. The two-clip bucket kept the same cup and the same coins.
• Mistake: planning wastes time. A plan shows what you need before you start, and it lets your team build the same thing.

What to carry forward

• The five steps are ask, imagine, plan, create and improve, and they go round in a loop.
• Engineering is Elementary, at the Museum of Science in Boston, made these five steps for children.
• Criteria say what a design must do. Constraints say what limits you must work inside.
• Brainstorm several ideas, then check each one against the criteria.
• Test, watch how the design fails, then change one thing and test again.
• When the situation changes, go back to Ask.

Sources

1. Berger, C. (2016, January 5). Get smart about the engineering design process with this fun, fast video. EiE Blog, Museum of Science, Boston. yesblog.mos.org
2. EiE Team. (2020, November 27). What is the engineering design process? EiE Blog, Museum of Science, Boston. yesblog.mos.org
3. Museum of Science, Boston. (n.d.). Engineering design process. Youth Engineering Solutions. yes.mos.org
4. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

Two strips of tape and a wrong answer

Two strips of tape hang over the edge of a table. One is masking tape. The other is clear tape. A paper cup hangs from each strip, and coins go into the cups one at a time until the tape lets go.

Imagine your friend Sam runs this test. The masking tape holds 14 coins before it peels off. The clear tape holds only 6. Sam says: "Masking tape is stronger than clear tape."

Is Sam right? Maybe. But look at how Sam did the test.

• Sam stuck 10 centimeters of masking tape to the table, but only 3 centimeters of clear tape.
• Sam pressed the masking tape down hard with a thumb. The clear tape was just laid on.
• The masking tape went on the clean kitchen table. The clear tape went on a dusty windowsill.
• The masking tape cup got small coins. The clear tape cup got big, heavy coins.
• Each tape was tested only once.

Now the answer is not clear at all. Did the masking tape win because it is stronger? Or because more of it was stuck down, or because it was pressed harder, or because the table was cleaner, or because its coins were lighter? There is no way to tell. Too many things changed at once.

The point: If more than one thing changes, you cannot tell which change made the difference.

What makes a test fair

A fair test is a test where you change only one thing on purpose and keep everything else the same. Then, if the result changes, you know what caused it.

Anything in a test that could change is called a variable (say it: VAIR-ee-uh-bul). Every fair test has three kinds of variable.

The science standards for grades 3 to 5 ask students to plan and carry out fair tests in which variables are controlled. Controlled is the grown-up word for kept the same. The same standards say good tests look for failure points, which are the places where a design gives way. You will look for those too.

How two brothers tested 200 wings

Orville and Wilbur Wright ran a bicycle shop in Dayton, Ohio. They also wanted to build a flying machine. In 1901 their glider flew badly. Sometimes it dived nose-first into the ground. The brothers decided to measure for themselves how wings behave in moving air.

In the autumn of 1901 they built a wind tunnel. It was a long, square tube with a fan that drove air through it. The part where the testing happened was 16 inches square, about the width of a big school notebook opened flat. Inside, they placed small model wings and measured how strongly the moving air lifted them.

In late October and early November they tested about 200 different wing shapes. Those first tests mostly helped them get the tunnel working well. Then, starting on 22 November, they tested 38 model wings carefully, each one at 45 different angles.

Here is why their results could be trusted. The wind was the same for every wing. The measuring was done the same way every time. Only the wing changed. That is a fair test.

Their tests showed that a number other people had used to work out lift was wrong. They used their own measurements to design their next glider. Two years later, in December 1903, they flew their powered airplane, the Wright Flyer, at Kitty Hawk in North Carolina. You will read about that day in Lesson 14.

Bottom line: Careful, fair tests let the Wright brothers trust their own numbers. Their machine flew because of what they measured.

Run the tape test fairly

Now fix Sam's test. You will compare four kinds of tape, such as masking tape, clear tape, duct tape and one more kind you have at home.

Here is what you need.

• Four kinds of tape
• A ruler and a pen
• Safety scissors
• 1 paper cup
• 3 paper clips
• About 50 coins, all the same kind
• A table with a smooth top that a grown-up says you may use
• A folded towel to put on the floor under the cup

Make the cup's handle first. Push one paper clip through each side of the rim, opposite each other. Hook both clips onto a third clip. This is the same bucket handle you made in Lesson 2.

1. Cut a strip of tape 15 centimeters long.
2. Mark a line 5 centimeters from one end. Stick that 5 centimeters flat on the table, close to the edge, so the other 10 centimeters hangs down over the edge.
3. Rub along the stuck part with your thumb three times. Rub the same way for every strip.
4. Fold the bottom of the hanging end around the top clip of your cup handle, and press the tape onto itself so the clip cannot slip out.
5. Add coins to the cup one at a time. Wait while you count to five after each coin.
6. When the tape lets go, count the coins in the cup. That is your result.
7. Write down how it failed. Did the tape peel off the table, tear in the middle, or slide off the clip?
8. Do three tries with each kind of tape, using a new strip each time.

Safety: Ask a grown-up before you stick tape to furniture, because some tapes can pull off paint or polish. Put the towel under the cup so falling coins do not land on toes. Keep the coins away from babies and toddlers.

Find the middle number

Three tries rarely give exactly the same answer. Maybe one strip was stuck down a little crooked. So which number do you use?

Put the three numbers in order from smallest to biggest, and pick the one in the middle. If your tries gave 12, 15 and 13 coins, the order is 12, 13, 15, so the middle number is 13. One odd result, high or low, cannot fool the middle number.

Here is a sample results table. Your numbers will be different.

Look at the clear tape row. The 4 is much lower than the other two tries. Maybe that strip had a crease. The middle number, 9, is not fooled by it. That is the reason for doing three tries.

Reading your table

A table of results is called data. Data is information you collected by measuring or counting. Engineers read data carefully before they decide anything.

In the sample, duct tape held the most coins. Does that make it the best tape? Only if holding coins is what you need. If you need tape that you can peel off a poster later without ripping it, the tape that let go easily might be the right one. The best choice depends on the criteria, which you met in Lesson 2.

The failure points matter too. The clear tape tore in the middle, which means the tape itself broke. The others peeled off the table, which means the sticky side let go first. Those are two different problems, and they would need two different fixes.

For the grown-up helping: Children often want to cross out a result they did not expect. Encourage them to keep every number and write a note beside the odd one instead. Honest records are the habit this lesson is building.

Good records

Write each result down the moment you get it, before you forget. Use the same unit every time: count coins as coins, measure length in centimeters, time in seconds. Put the date at the top of the page. Write down anything odd that happened, like a draft from an open window or a crease in the tape.

If you want to show your results as a picture, draw a bar chart in your notebook. Make one bar for each tape, as tall as its middle number. The tallest bar shows which tape held the most coins.

Common misconceptions

• Mistake: one test is enough. A single try can be fooled by an accident, such as a crease in the tape. Three tries and a middle number are much harder to fool.
• Mistake: changing two things at once saves time. It wastes time, because afterwards you cannot tell which change made the difference.
• Mistake: a result you did not expect must be a mistake. Sometimes it is. Sometimes it is the most interesting thing you found. Keep it, and write a note beside it.
• Mistake: fair means everyone gets a turn. In engineering, a fair test means only one thing changes and everything else stays the same.
• Mistake: the biggest number is always the winner. The best design is the one that meets the criteria. Sometimes that is the smallest number, such as the shortest time.

Recap

• In a fair test you change one thing, keep everything else the same, and measure the result.
• Anything that can change in a test is a variable. Kept the same is called controlled.
• Do three tries and use the middle number, so one odd result does not fool you.
• Write results in a table straight away, with units, and note how each design failed.
• The Wright brothers tested about 200 wing shapes in their 1901 wind tunnel, changing only the wing.
• The best design is the one that meets the criteria, not always the one with the biggest number.

Sources

1. U.S. Centennial of Flight Commission. (n.d.). Further gliding and wind tunnel experiments, 1901. centennialofflight.net
2. Smithsonian National Air and Space Museum. (n.d.). 1903 Wright Flyer. Collection objects. airandspace.si.edu
3. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org
4. NGSS Lead States. (2013). K-2-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

Three pigs and three materials

In 1890, a writer called Joseph Jacobs printed an old story in a book called English Fairy Tales. You may know it. Three little pigs leave home, and each one builds a house.

In Jacobs's book, the first pig builds a house of straw. The second builds a house of furze, which is a spiky bush. The third builds a house of bricks. Then a wolf comes to each door and says the famous words: "Little pig, little pig, let me come in." When the pig refuses, the wolf says: "Then I'll huff, and I'll puff, and I'll blow your house in."

The straw house falls down. The furze house falls down. The brick house stands.

It is a story, not a science report. But it carries a real engineering lesson. All three pigs had the same job: build a house that keeps the wolf out. Only one of them picked a material that was right for the job.

What matters here: Choosing the material is part of the design. A good shape made from the wrong material can still fail.

What a property is

A material is the stuff a thing is made from: paper, wood, metal, plastic, cloth, glass. A property is something about a material that you can observe or test. Is it strong? Is it stiff or bendy? Does it let water through? Does it soak up water?

Here are the properties you will use most in this course.

• Strong: it does not break or tear easily when you pull it or press on it.
• Stiff: it keeps its shape and does not bend much.
• Flexible: it bends easily without breaking. Flexible is the opposite of stiff.
• Waterproof: water cannot get through it.
• Absorbent (say it: ab-ZOR-bent): it soaks up water, like a sponge.
• Elastic: it stretches and then springs back, like a rubber band.

The science standards for second grade ask children to test different materials and decide which ones have the properties best suited to a job. The examples they give are strength, flexibility, hardness, texture and absorbency. The big idea behind the standard is short: different properties are suited to different purposes.

The materials table

Here are the materials in an engineer's kitchen-table toolbox, and what you will usually find when you test them. Do not just believe the table. Test a few of them yourself.

Corrugated cardboard is the brown board that delivery boxes are made of. Look at a cut edge. You will see a wavy layer of paper glued between two flat layers. That wavy layer is what makes it so stiff.

Reading the table

Look down the table and notice three things.

First, no material is best at everything. Foil is waterproof but tears easily. String is very strong when you pull it, but you cannot push with it. Try pushing a piece of string across a table. It just crumples. So string is perfect for a zip line, and useless as a tower leg.

Second, some materials change when they get wet. Paper and cardboard go soft and weak when they soak up water. A cardboard boat might float for a minute, and then sag.

Third, shape changes a material's strength. A flat sheet of paper flops. Roll the same sheet into a tube and it can hold up a book. You will test this in Lessons 5 and 8.

In short: Pick the material whose properties match the job. Then think about what shape it should be.

Test 1: the toy umbrella test

Which material keeps a toy dry in the rain? Here is a fair test you can do on a plate.

You need: squares of five materials, each 10 centimeters by 10 centimeters (try printer paper, paper towel, aluminum foil, plastic wrap and a scrap of cotton cloth); five tissues or squares of paper towel; a plate; a cup of water; a drinking straw; a ruler; and a timer or a clock.

First, make a dropper from the straw. Dip the straw into the water. Put your finger tightly over the top end, and lift the straw out. The water stays inside. Hold it over the place you want, lift your finger, and the water drops out. That is one strawful.

1. Lay a dry tissue on the plate. This is your toy. You want to keep it dry.
2. Lay one test material flat on top of the tissue.
3. Drop three strawfuls of water onto the middle of the material, from the same height each time.
4. Wait one minute.
5. Lift off the material. Measure the widest part of any wet patch on the tissue with your ruler, in centimeters. If the tissue is dry, write 0.
6. Use a fresh dry tissue for the next material. Test all five.

Ask yourself what you kept the same: the amount of water, the height, the waiting time and the kind of tissue. The only thing you changed was the cover material. That makes it a fair test.

Here is a sample of what a results table can look like. Your numbers will be different.

The waterproof materials keep the toy dry. The absorbent ones let water through. A paper towel is a terrible umbrella, but it is a great mop. The property that makes it fail one job makes it perfect for another.

Test 2: the droop test

How stiff is each material? This test measures stiffness with a ruler.

1. Cut a strip of each material 2 centimeters wide and 20 centimeters long. Try printer paper, card, corrugated cardboard and foil. A grown-up cuts the cardboard strip.
2. Lay the strip on the table so that exactly 10 centimeters sticks out over the edge.
3. Hold the part on the table down flat with a heavy book.
4. Measure how far the end of the strip has drooped below the tabletop, in centimeters.

The smaller the droop, the stiffer the material. You will probably find that the cardboard hardly droops at all and the paper flops down. Stiffness matters a lot for bridges and shelves, which you will build soon.

Choosing for the job

Here is how an engineer thinks when choosing a material. Start from the job, then look for the property it needs.

Your engineering toolbox

Start a box of free building materials now. Ask a grown-up before you take anything, and keep it clean and dry.

• Cereal boxes, other thin card, and corrugated cardboard boxes
• Paper towel tubes and toilet paper tubes
• Clean plastic bottles and bottle caps
• Paper cups, drinking straws, string, paper clips and rubber bands
• Old magazines and newspapers
• Tape, safety scissors, a ruler, a pencil, and a jar of coins to use as weights

Workshop safety rules

These rules apply to every build in this course. Each lesson will remind you of the ones it needs.

1. Scissors: use safety scissors. Carry them closed, with the points down. Cut away from your fingers.
2. Craft knives: a craft knife or box cutter is for grown-ups only. A grown-up cuts card and cardboard, on a cutting board or an old magazine.
3. Hot glue: only with a grown-up. The tip of a glue gun and the melted glue are hot enough to burn skin. Tape and white glue do most jobs in this course.
4. Eyes: anything that flies, flings or snaps can hurt an eye. Never aim anything at a person or a pet. If you have safety glasses, wear them for launches.
5. Small parts: coins, marbles, paper fasteners and bottle caps can choke a baby or toddler. Keep them away from younger brothers and sisters, and tidy them away after every build.
6. Water: wipe up spills straight away, and keep water away from anything that plugs in.
7. Plastic wrap: keep it, and plastic bags, away from babies and toddlers.

For the grown-up helping: The U.S. Consumer Product Safety Commission bans small parts in toys for children under three, because a small object can block a young child's airway. The same choking risk applies to the coins, marbles and fasteners used as building materials in this course, so a quick tidy after each session matters.

Common misconceptions

• Mistake: heavy means strong. A lump of clay is heavy but squashes easily. A drinking straw is light but surprisingly stiff along its length. Weight and strength are different properties.
• Mistake: the strongest material is always the best. The best material is the one that suits the job. A paper towel fails as an umbrella and wins as a mop.
• Mistake: paper is weak, so it is useless for building. Flat paper is weak. Folded or rolled paper can be strong. Shape matters as much as material.
• Mistake: waterproof things do not get wet. Water still lands on foil. Waterproof means the water cannot get through to the other side.

The short version

• A material is what a thing is made of. A property is something about it you can observe or test.
• Useful properties include strong, stiff, flexible, waterproof, absorbent and elastic.
• Different properties suit different jobs, and no material is best at everything.
• Fair tests, like the drip test and the droop test, show which material suits a job. Measure in centimeters.
• Shape changes strength: rolled or folded paper is much stronger than a flat sheet.
• Follow the workshop safety rules every time.

Sources

1. Jacobs, J. (Ed.). (1890). The story of the three little pigs. In English fairy tales. David Nutt. Project Gutenberg
2. NGSS Lead States. (2013). 2-PS1-2 Matter and its interactions. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org
3. U.S. Consumer Product Safety Commission. (n.d.). Small parts and choking hazard labeling FAQs. cpsc.gov
4. U.S. Consumer Product Safety Commission. (n.d.). Small parts for toys and children's products. Business guidance. cpsc.gov

A square that will not stay square

Lay four strips of card on a table and join their corners with four paper fasteners, so that they make a square. Now push one corner sideways with one finger.

The square does not stay a square. It leans over into a diamond shape, and if you keep pushing, it folds almost flat. You hardly had to push at all.

Now make a triangle from three strips and three fasteners, and push one of its corners in the same way. The whole triangle slides across the table, but it stays a triangle. It does not fold.

Same card. Same fasteners. Same push. So why does one shape fold and the other hold? That is the question this lesson answers. The answer explains bridges, cranes, roofs, bicycles and towers.

What the fasteners are doing

A paper fastener is a small metal pin with two flat legs. You push it through two holes and open the legs flat on the back. The two strips are joined, but they can still turn around the pin, like the hands of a clock turn around the middle.

The place where two strips meet is called a joint. When a joint can turn, the angle at that corner can change. An angle is how wide open a corner is. A square's corners are right angles, like the corner of a book.

In the square, all four corners can turn. When you push, the corners change from right angles into two wide ones and two narrow ones. Not one strip gets longer or shorter. The strips just tilt, and the square becomes a diamond.

Why a triangle holds

Here is the trick. If you know how long the three sides of a triangle are, there is only one triangle you can make with them. Try it with three pencils of different lengths: however you arrange them, the corners always come out the same.

So the only way to change a triangle's corners is to change the length of a side. You would have to squash a side shorter, stretch it longer, bend it or break it. Card does not do that easily. So the triangle keeps its shape.

Four sides are different. Four sticks of the same length can make a square, a tall thin diamond or a flat diamond. Nothing locks the corners in place.

Engineers who teach this explain it the same way. A square can be pushed until it is no longer a square, while a triangle's angles are fixed by the lengths of its sides. That is why a triangle cannot collapse unless one of its sides gives way.

Key idea: A triangle's three sides lock its corners in place. That is why engineers build with triangles.

Squash and stretch

When you push or pull a frame, each strip feels one of two kinds of force.

• Compression (say it: com-PRESH-un) is a squashing force. It tries to make a thing shorter. Press the two ends of a sponge together and you are compressing it.
• Tension (say it: TEN-shun) is a stretching force. It tries to make a thing longer. A rope in a tug of war is in tension.

When you push the top corner of a triangle sideways, one sloping side gets squashed and the other gets stretched. Neither can change length easily, so the triangle holds.

Here is something you will see in your test. Thin card is much stronger when it is stretched than when it is squashed. Pull on the ends of a card strip and it is very hard to break. Push its ends together and it bends in the middle. That bend is a failure point, and you will meet it again.

Bracing a square

What if you need a square shape, like a doorway or a window? Engineers add a brace: an extra strip that runs across the square from one corner to the opposite corner.

Now look at the shape. The brace has cut the square into two triangles. Each triangle locks its own corners, so the whole square is locked. A square with a brace across it holds its shape.

Look around for braces. Wooden gates often have a slanting board across them. Many shelves have a slanting strip at the back. The A-shaped legs of a swing set make big triangles. A bicycle frame is made of two triangles joined together.

A frame built from triangles is called a truss. Truss bridges, roof frames, cranes and electricity towers are all built from trusses. Look at a photo of a big crane or a power line tower and count the triangles.

Worth holding on to: To make a shape stiff, you do not always need more material. One brace in the right place turns a floppy square into two strong triangles.

Build the frames

You need:

• Thin card, such as an empty cereal box
• A ruler and a pencil
• Safety scissors
• 12 paper fasteners
• A hole punch, or a grown-up with a sharp pencil and a ball of play dough
• Tape, a sheet of paper, 50 centimeters of string, 1 paper cup and 3 paper clips
• About 40 coins, all the same kind

1. Cut seven strips of card, each 2 centimeters wide and 15 centimeters long.
2. Mark a dot 1 centimeter in from each end of every strip.
3. A grown-up makes a hole at each dot. A hole punch works well. Or lay the strip on the play dough and push a sharp pencil through.
4. Make a triangle: join three strips end to end with three fasteners. Open each fastener's legs flat on the back.
5. Make a square: join four strips with four fasteners.
6. For the brace, cut a longer strip, 2 centimeters wide and 22 centimeters long. Lay it across the square from one corner to the opposite corner. Mark where its holes need to go, right over the two corner holes. Make the holes, and push the corner fasteners through the brace too.

Safety: The legs of a paper fastener are sharp at the tips, so open them flat and keep them away from eyes. Only a grown-up uses the sharp pencil. Fasteners and coins are small parts, so keep them away from babies and toddlers, and count them back into a box when you finish.

The sideways pull test

This test pulls the top corner of each frame sideways with a cup of coins, and counts how many coins it takes to make the frame give way.

1. Lay the sheet of paper on the table, close to one end. Lay the frame flat on the paper, with its bottom strip running along the table.
2. Tape the bottom strip down firmly with two pieces of tape.
3. Draw around the frame with a pencil, so you can see if it moves.
4. Tie the string to the top corner on the side nearest the end of the table. Run the string straight sideways to the end of the table, and let it hang over the edge.
5. Make a bucket handle on the cup with the three paper clips, as in Lesson 2, and hang the cup on the string.
6. Add coins one at a time. Stop when the top corner has moved 2 centimeters from its pencil line, or when a strip bends.
7. Write down the number of coins, and what gave way.

Test the triangle, the square, and the braced square. Keep everything else the same: the same tape, the same string, the same cup, the same coins. Then turn the braced square over so the string pulls the other way, and test it again.

Here is what you might see. Your numbers will be different.

What the table tells you

The square folded with almost no load. Every triangle shape held many times more.

The braced square is the interesting one. When the pull stretched the brace, it held well. When the pull squashed the brace, the long thin strip bent and the square began to fold. Card is strong when stretched and weak when squashed. That is a failure point, and the table found it.

So improve it. Add a second brace across the other two corners, making an X. Now, whichever way you pull, one of the braces is being stretched, and that brace holds the frame. Real buildings and towers often use X-shaped braces for the same reason.

For the grown-up helping: If the frames slide instead of folding, the tape on the bottom strip needs pressing down harder. The point of the test is the big difference between the square and the triangle shapes, not the exact numbers.

Triangles in three dimensions

The same idea works for solid shapes. Build a cube from 12 drinking straws, joining the corners with tape or paper clips. Press on its top and it leans over, just like the flat square. Now build a pyramid from 6 straws, with a triangle at the bottom and three straws meeting at the top. Every face of that pyramid is a triangle, and it stands firm.

Common misconceptions

• Mistake: triangles are strong because their corners are pointy. Pointy corners have nothing to do with it. A triangle holds because its three sides lock its angles in place.
• Mistake: a square is weak because it has more sides. The problem is that its corners can turn without any side changing length. Brace it into triangles and it holds.
• Mistake: triangles can never break. A side can still bend, stretch or snap if the load is big enough. In the test, the squashed side was the failure point.
• Mistake: to make something stronger, just use more material. One brace in the right place did more than doubling the card would. Engineers look for the smart shape first.

What you now know

• A square with turning joints folds into a diamond, because its corners can change without any side changing length.
• A triangle keeps its shape, because its three sides lock its corners in place.
• Compression is a squashing force. Tension is a stretching force.
• Thin card is much stronger when stretched than when squashed.
• A brace turns a square into two triangles. Two braces in an X hold whichever way you push.
• A frame made of triangles is called a truss. Bridges, cranes, roofs and bicycles use them.

Sources

1. Chinnis, D., Guiliani, A., Duckworth, S., and Zarske, M. S. (2013). Triangles and trusses. TeachEngineering. Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder. teachengineering.org
2. Goode, J. S., Zarske, M. S., and colleagues. (2007). Bridging the gaps. TeachEngineering. Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder. teachengineering.org
3. NGSS Lead States. (2013). K-2-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

Eighty thousand miles of wire

Each of the two main cables of the Golden Gate Bridge in San Francisco is made of 27,572 steel wires bundled together. Each wire is about 5 millimeters thick, a little thinner than a pencil. Put end to end, the wire in the two cables would stretch 80,000 miles. That is more than three times around the Earth.

The bridge was finished in 1937. Its main span, the part between its two towers, is 4,200 feet long. Its towers stand 746 feet above the water.

Why so much wire? Because the road of the Golden Gate Bridge hangs from those two cables. Every car, truck and bus crossing the bridge is held up by wires being pulled.

That is one way to build a bridge. There are others. In this lesson you will compare four kinds, then build paper bridges and test them.

Every bridge has the same job

A bridge carries a load across a gap. The load is everything the bridge must hold up: its own weight, plus the people, cars or trains on it. The bridge has to send all that weight safely to the ground at each end.

Remember the two forces from Lesson 5. Compression squashes. Tension stretches. Engineers who teach bridge design say that two forces act on every bridge, all the time: compression and tension. The different kinds of bridge are different ways of handling them.

Four kinds of bridge

Reading the table

Beam bridges are the simplest. A log across a stream is a beam bridge. Stand in the middle and the log bends a little. Its top gets squashed and its bottom gets stretched. A beam bridge works well across a short gap, but a long, flat beam sags.

Truss bridges fix the sagging problem with triangles. You saw in Lesson 5 that triangles keep their shape. A truss bridge uses a frame of triangles, so it stays stiff without needing a huge solid beam.

Arch bridges turn the load into a squash. Stand on top of an arch and your weight pushes along the curve, down into the ground at each end. Stone is very strong when squashed, so arches can be built from stone. Some of the strongest and longest-lasting bridges in the world are stone arches built in the time of the Roman Empire.

Suspension bridges turn the load into a stretch. The road hangs from cables. The cables run over tall towers and are tied down at each end. Steel wire is very strong when pulled, so suspension bridges can cross the widest gaps. The Golden Gate Bridge is one. So is the Brooklyn Bridge from Lesson 1.

The upshot: Each kind of bridge is a way of sending the load to the ground, by squashing parts that are good at being squashed and stretching parts that are good at being stretched.

Build paper bridges

Now test some bridge ideas with paper. Every bridge will cross the same gap and carry the same coins.

You need:

• Two stacks of books the same height, about 10 centimeters tall
• A ruler
• At least five sheets of printer paper, all the same size
• Tape and safety scissors
• A small paper cup
• About 100 coins, all the same kind

Put the two stacks of books on the table with a gap of exactly 20 centimeters between them. Measure the gap before every test.

Build these four bridges, one at a time, each from one sheet of paper.

1. The flat beam. Lay a flat sheet across the gap.
2. The folded edges. Fold up 2 centimeters along each long side of the sheet, so it looks like a long tray. Lay it across the gap with the folded edges standing up.
3. The zigzag. Fold the sheet backward and forward along its length, like a paper fan, with folds about 2 centimeters apart. Lay it across the gap so the folds run from one stack of books to the other.
4. The arch. Take a second sheet and bend it into a curve that fits under the gap, with its ends pushed against the books. Lay a flat sheet across the top of it.

The coin test

1. Stand the paper cup in the middle of the bridge.
2. Add coins one at a time.
3. Stop when the bottom of the cup touches the table, or when the bridge falls into the gap.
4. Count the coins in the cup. That is the result.
5. Write down how the bridge failed. Did it sag, buckle, slide off a book, or tear?
6. Test each bridge three times with fresh paper, and use the middle number.

Keep the gap, the paper, the cup and the coins the same. Change only the design. That is your fair test.

Safety: Keep toes and fingers clear of the gap, and do not build the book stacks so high that they can topple. Coins are small parts, so keep them away from babies and toddlers.

Here is a sample of what you might find. Your numbers will be different.

What the paper bridges show

The flat sheet failed with almost nothing on it. The same sheet, folded, held dozens of coins. The paper did not get stronger. Its shape changed.

Folding makes the paper deeper. Deep means tall from top to bottom. A flat sheet is only as deep as the paper is thick. A zigzag is as deep as its folds are tall. A deep beam is much harder to bend than a flat one. That is why the steel beams in buildings and bridges are usually tall and narrow, standing on their edges, rather than laid flat like a plank. It is also why corrugated cardboard, with its wavy middle layer, is so stiff.

The arch held more than the flat sheet, but look at how it failed. Its ends slid apart along the table. An arch pushes outward at its ends, so it needs something solid to push against. Real arch bridges are built into strong banks of rock or concrete for exactly this reason. Try taping the arch's ends to the books, or pushing a heavy book against each end, and test again. That is the Improve step.

Remember: Shape can matter more than material. A fold, a curve or a triangle can turn a floppy sheet into a strong bridge.

The bridge that twisted

Bridges must stand up to wind as well as weight. In 1940, engineers learned this the hard way.

The Tacoma Narrows Bridge in Washington State opened on 1 July 1940. It was a suspension bridge, long and narrow. Its road moved up and down in the wind, and people nicknamed it Galloping Gertie.

Before the collapse, a professor called F. Bert Farquharson built a small model of the bridge and tested it in a wind tunnel, like the Wright brothers' tunnel in Lesson 3. He saw the model twist, and he warned that if the real bridge ever twisted like that, it would be the end of the bridge. He thought the solid steel sides of the road were the problem, because the wind could not pass through them. Repairs were still being planned.

On 7 November 1940, a strong wind of about 40 miles an hour blew along the Narrows. The road began to twist, tipping up on one side and then the other, until it tilted halfway onto its side. About an hour later, the middle of the bridge tore apart and fell into the water.

No people died. The people on the bridge got off in time. Sadly, a dog called Tubby, who was shut in a car on the bridge, was lost. The Washington State Department of Transportation says the collapse changed forever how engineers design suspension bridges. A new, much safer Tacoma Narrows Bridge opened in 1950.

For the grown-up helping: Film of the 1940 collapse is easy to find online and is striking to watch. It is worth previewing first with a sensitive child. The point to draw out is that the model test predicted the problem, and engineers now take wind seriously from the start.

Common misconceptions

• Mistake: a stronger material is the only way to make a stronger bridge. In your test, the same paper held dozens more coins when folded. Shape matters as much as material.
• Mistake: a bridge only has to hold up the cars on it. It must hold its own weight, the traffic, and the push of the wind. Tacoma Narrows held its traffic but failed in the wind.
• Mistake: arches pull things together. An arch is squashed, and it pushes outward at its ends. That is why it needs strong banks to push against.
• Mistake: suspension bridges are held up from below. Their roads hang from cables above. The cables are stretched, and the towers carry the cables.

Putting it together

• Every bridge carries its load across a gap to the ground, using compression and tension.
• Beam bridges bend: the top squashes and the bottom stretches.
• Truss bridges use triangles to stay stiff.
• Arch bridges are squashed and push outward into their banks.
• Suspension bridges hang from stretched cables over squashed towers, like the Golden Gate Bridge.
• Folding paper makes it deeper, and a deeper beam is much harder to bend.
• The Tacoma Narrows Bridge twisted apart in the wind in 1940, and engineers changed how they design suspension bridges.

Sources

1. Golden Gate Bridge, Highway and Transportation District. (n.d.). Design and construction statistics. goldengate.org
2. Goode, J. S., Zarske, M. S., and colleagues. (2007). Bridging the gaps. TeachEngineering. Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder. teachengineering.org
3. Washington State Department of Transportation. (n.d.). Collapse. Tacoma Narrows Bridge history. wsdot.wa.gov
4. Washington State Department of Transportation. (n.d.). Tacoma Narrows Bridge history. wsdot.wa.gov

The tallest building in the world

The Burj Khalifa in Dubai was finished in 2010. It is 828 meters tall and has 163 floors above the ground. If you could lay it down on its side, it would stretch about nine American football fields, goal line to goal line.

A building that tall has three jobs. It must hold up its own enormous weight. It must not tip over. And it must stand firm when the wind pushes on it, hour after hour.

Its engineers at a company called SOM gave it a special shape. Seen from above, the tower is shaped like the letter Y, with three wings around a six-sided middle. The designers say this simple Y shape reduces the push of the wind. Each wing helps hold up the others.

As it rises, the tower steps inward again and again in a spiral, so it gets thinner toward the top. That puts less of its weight high up. The designers say the steps confuse the wind. Each level has a slightly different shape, so swirls of wind cannot build up into a strong, steady rhythm.

Your tower will be made of paper, not steel and concrete. But it will face the same three jobs: hold its weight, do not tip, and survive the wind.

The balance point

Every object has a center of gravity. NASA describes it as the average location of the weight of an object. An easier way to think of it is the balance point: the place where the object would balance perfectly if you rested it on a fingertip.

Try it with a ruler. Rest it across one finger and slide your finger along until the ruler balances. For a ruler, the balance point is right in the middle. Now tape a few coins to one end and find the balance point again. It has moved toward the coins.

Here is the rule that decides whether a tower tips. A thing stays standing as long as its balance point is above its base, the part touching the ground. Once the balance point leans out past the edge of the base, the thing tips over.

That gives two ways to make a tower harder to tip.

1. Make the base wider. A wide base means the balance point has a long way to lean before it passes the edge. Stand with your feet together and get a friend to push your shoulder gently. Then stand with your feet apart. Much harder to push over.
2. Keep the weight low. A low balance point has to tilt much further before it leans past the edge. Bending your knees lowers your balance point, which is why sports players crouch when they want to be hard to knock over.

In short: Wide at the bottom and heavy at the bottom means hard to tip. Tall, thin and top-heavy means easy to tip.

The challenge

Build the tallest tower you can that stands up by itself for 10 seconds. Then see whether it is also the hardest to knock down.

You need:

• 10 sheets of printer paper
• 1 meter of tape
• Safety scissors, a pencil and a tape measure
• A large hardback book or a tray, for the tilt test
• A few coins
• Optional: an electric fan, set up by a grown-up

Your constraints are the 10 sheets, the 1 meter of tape and 20 minutes of building time.

Most paper towers start with tubes. Roll a sheet tightly around a pencil and tape the edge. Rolling from one corner to the opposite corner gives a longer tube than rolling from one side. Slide the pencil out. A tube is much stiffer than the flat sheet, just like the folded bridges in Lesson 6.

Worked example: two towers

Here is how two designs might do. Your numbers will be different.

Tower A, the Skinny. Four long tubes taped end to end, standing on a small cross of folded paper at the bottom. It goes straight up.

Tower B, the Tripod. Three tubes lean together and are taped at the top, like the poles of a camping tripod. Three short tubes are taped across between the legs near the bottom, making triangles. Two more tubes stand on top of the tripod.

Test 1 is the height test. Measure from the table to the very top, and check that each tower stands for 10 seconds.

If height were the only criterion, Tower A would win easily.

A new test flips the result

Now add a second criterion: the tower must not tip easily. Here is the tilt test.

1. Stand the tower on the large book, near one end.
2. Tape a pencil across the book just downhill of the tower, so the tower cannot slide off. It can only tip.
3. Slowly lift the other end of the book.
4. The moment the tower starts to tip, stop. Measure how high the lifted end of the book is above the table, in centimeters.
5. Do it three times and use the middle number.

The higher you can lift the book before the tower tips, the more stable the tower is.

If you have a fan, a grown-up can do the fan test. Set the fan on its lowest speed, 1 meter from the tower, and switch it on for 10 seconds. If the tower stands, move the fan 25 centimeters closer and try again. Record the closest distance at which the tower still stands.

Now Tower B wins both new tests. It is shorter, but its wide base and its triangles make it far more stable. The result flipped because the criteria changed.

So what? Which design is best depends on what you test. A real tower must be tall enough and stable enough, so engineers test for both.

Improve the Skinny

Can Tower A be saved? Use what you know about balance points.

• Widen the base. Tape three short tubes to the bottom as legs, slanting outward. Each leg and the tower make a triangle.
• Lower the balance point. Drop five coins inside the bottom tube, so there is extra weight low down.

Test again, changing those things only. In the example, the improved Skinny stood 90 centimeters tall and tipped at 6 centimeters instead of 2. It is still not as stable as the Tripod, but it is much better, and it is taller. Engineers often have to trade one criterion against another like this.

Safety: Only a grown-up sets up and moves the fan. Keep fingers, hair and paper away from the fan blades, and never poke anything through the guard. Do not stand on a chair to measure a tall tower; ask a grown-up to measure it. Coins are small parts, so keep them away from babies and toddlers.

For the grown-up helping: The tilt test works best on a stiff, flat surface. If the tower slides instead of tipping, the pencil stop needs to sit right against its base. Expect wide variation between attempts; the middle number of three is the fairest summary.

Common misconceptions

• Mistake: the tallest tower is the best tower. The best tower meets all the criteria. A tall tower that falls in a breeze has failed.
• Mistake: a heavier tower is always harder to knock over. Where the weight sits matters more. Weight low down makes a tower stable. Weight at the top makes it easier to tip.
• Mistake: a wide base wastes material. A wide base is what keeps the balance point above the base when the tower is pushed. It is money, or paper, well spent.
• Mistake: skyscrapers are simple boxes. The Burj Khalifa's Y shape and spiral steps were chosen partly to handle the wind.

Summing up

• The Burj Khalifa, finished in 2010, is 828 meters tall. Its Y shape and its steps help it handle the wind.
• The center of gravity, or balance point, is the average location of an object's weight.
• A tower stays up while its balance point is above its base. A wide base and low weight make it harder to tip.
• Tubes and triangles make a paper tower stiff.
• The tilt test measures stability in centimeters. A new test can change which design wins.
• Engineers test for every criterion, not just the one that is easiest to measure.

Sources

1. Council on Tall Buildings and Urban Habitat. (n.d.). Burj Khalifa. The Skyscraper Center. skyscrapercenter.com
2. Skidmore, Owings and Merrill. (n.d.). Burj Khalifa. Projects. som.com
3. NASA Glenn Research Center. (2024, July 17). Center of gravity. Beginner's guide to aeronautics. nasa.gov
4. Urone, P. P., and Hinrichs, R. (2022). 9.3 Stability. In College physics 2e. OpenStax. openstax.org

Beds made of cardboard

In September 2019, the organizers of the Tokyo 2020 Olympic and Paralympic Games announced something surprising. The beds in the villages where the athletes would sleep would have frames made of cardboard.

This was not ordinary cardboard. It was very strong cardboard, and each bed frame could hold up to 200 kilograms. That is about as heavy as three grown-ups. After the Games, the frames would be recycled into paper products.

If cardboard can hold up an Olympic athlete, can paper hold up a pile of books? In this lesson you will find out by building a chair from paper and tape. You will start with a chair that fails, and you will find out exactly why.

The challenge

Build a chair for a teddy bear or a doll, using only paper and tape. Its seat must be at least 10 centimeters above the table. Then see how many books it can hold on its seat.

You need:

• 6 sheets of printer paper or newspaper
• 1 meter of tape
• Safety scissors, a ruler and a pencil
• A square of card about 15 centimeters across, to lay on top when testing
• About 10 paperback books, all roughly the same size
• A folded towel to work on

Safety: A paper chair is for toys and books, never for people or pets. Test it on a towel on the floor or a low table, and keep your toes out from under the books. If you use cardboard, a grown-up does any cutting with a craft knife.

Chair 1: a chair that fails

Here is a first try that many people make. Each of the four legs is a strip of paper folded in half the long way. The seat is one flat sheet of paper, taped to the tops of the legs.

Put one book on the seat. The seat sags in the middle. The legs bend sideways and twist. In a few seconds, the whole chair folds flat under the book.

It would be easy to say paper is just too weak. But you already know that is not true: folded paper held dozens of coins in Lesson 6. So let us trace the failure, one fault at a time, the way an engineer would.

Fault 1: the seat sagged

The seat was one flat sheet. You learned in Lesson 6 that a flat sheet is not deep, so it bends easily. The fix is to make the seat deeper. Fold the seat sheet into a zigzag, or roll several tubes and tape them side by side like a raft. Either way, the seat stops sagging.

Fault 2: the legs bent

A chair leg is a column: a post that holds up a load above it. Science North, a science center in Canada, explains that columns are very good at resisting pushing forces, which you know as compression. But columns can fail in two ways. They can be crushed, or they can buckle, which means bend suddenly sideways under a squash.

A strip of paper folded in half buckles almost at once. A rolled tube is far better. You can test which column shape is best before you build the chair.

The column test

Make three columns, each from one sheet of printer paper, each standing as tall as the short side of the sheet.

• Round column. Roll the sheet into a tube about as wide as a toilet paper tube, and tape the edge.
• Square column. Fold the sheet into four equal strips, then fold it round into a square tube and tape the edges together.
• Triangle column. Fold the sheet into three equal strips, then fold it round into a triangular tube and tape the edges together.

Now test each column the same way.

1. Stand the column on the towel.
2. Lay the square of card on top.
3. Add books one at a time, placing each one carefully in the middle.
4. When the column crumples, stop. Count the books it held before the last one.
5. Do three tries for each shape with fresh paper, and use the middle number.

Keep the paper, the height, the tape, the card and the books the same. Change only the shape. Here is a sample. Your numbers will be different.

The round column usually wins, and Science North reports the same thing in its classroom test: the square and triangle columns collapsed under a can of soup, while the round one held. Their explanation is about the middle of the load. When a load sits right in the middle of a round column, every part of the wall is the same distance from the middle. No part is weaker than the rest. A square or a triangle has corners and flat sides, and a flat side bends outward first. That is why buildings so often use round columns.

The core of it: A round column shares the load evenly around its wall, so it is less likely to buckle than a column with corners and flat sides.

Fault 3: the legs splayed and twisted

Even with tube legs, a chair can lean over, because its legs can tilt at the joints, like the square frame in Lesson 5. The fix is the same: braces. Tape a tube across between each pair of legs, low down, and add a slanting tube from each leg up to the seat. Each brace makes a triangle, and triangles hold their shape.

Fault 4: one leg took all the weight

If one leg is a little longer than the others, or the books sit near one edge, that leg takes most of the load and buckles first. Science North notes that a load that is not balanced on top of a column pushes harder on one side, which can make the column buckle. So make all four legs exactly the same length, and put the load in the middle of the seat. Then every leg takes a fair share.

Fault 5: the legs were too tall and thin

Science North also explains that shorter, thicker columns resist bending better than long, narrow ones. Your seat only needs to be 10 centimeters high, so do not use full-length tubes. Cut each tube down, or roll a sheet around itself several times to make a thicker wall.

Chair 2: the improved chair

Put every fix together.

1. Roll four tubes for legs. Cut them all to exactly 10 centimeters.
2. Roll four more tubes and tape them between the legs, low down, so each pair of legs is braced.
3. Fold a sheet into a zigzag for the seat, or tape tubes side by side into a raft, and tape it firmly to the tops of the legs.
4. Lay the square of card on the seat to spread the load, and put the books in the middle.

Here is how the improvements might add up. Again, your numbers will be different.

Notice that each row changes one thing from the row above. That way you can see which fix helped and by how much.

Key idea: A strong chair is the sum of small fixes: deep seat, round legs, equal lengths, short columns, and triangles between the legs.

For the grown-up helping: Paperback books of similar size make a fairer load than a mix of books. If you have a kitchen scale, weigh the stack your child's best chair held, and compare it with the 200 kilograms of the Tokyo bed frames.

Common misconceptions

• Mistake: more legs always make a stronger chair. Legs only help if they share the load. If one leg is longer than the rest, it takes most of the weight and buckles first.
• Mistake: paper is far too weak to hold books. A flat sheet is weak, but a rolled column of the same paper can hold several books.
• Mistake: a column fails by being squashed flat. A thin column usually buckles first, bending suddenly sideways long before it is crushed.
• Mistake: taller legs are stronger legs. Shorter, thicker columns resist buckling better than tall, thin ones.

What to remember

• The Tokyo 2020 athletes' bed frames were made of strong cardboard and could hold up to 200 kilograms.
• A column is a post that holds up a load. Columns usually fail by buckling: bending sideways under a squash.
• A round paper column usually holds more than a square or triangular one, because it has no weak corners or flat sides.
• Short, thick columns resist buckling better than tall, thin ones.
• A load in the middle lets every leg take a fair share.
• Deep seats and braces between the legs fix sagging and twisting.

Sources

1. International Paralympic Committee. (2019, September 27). Tokyo 2020: Innovative bedding for Paralympic and Olympic Villages. paralympic.org
2. Science North. (n.d.). Column challenge, grade 3: Strong and stable structures [Lesson plan]. sciencenorth.ca
3. Michigan State University Extension. (2022). The strongest column [Heads in, hearts in engineering activity, 4H1749EN]. Michigan 4-H Youth Development. extension.purdue.edu
4. Chinnis, D., Guiliani, A., Duckworth, S., and Zarske, M. S. (2013). Triangles and trusses. TeachEngineering. University of Colorado Boulder. teachengineering.org

A lid that will not budge

A can of paint comes with its metal lid jammed on tight. Pull at the edge with your fingertips and nothing happens. Now a grown-up slides the tip of a flat screwdriver under the rim, rests it on the edge of the can, and pushes down gently on the handle. The lid pops up.

Nobody got stronger. The screwdriver turned a small push into a big lift. When a stiff bar is used like this, it is called a lever, and it is one of the oldest and simplest machines there is.

A simple machine is a tool with few or no moving parts that changes the size or direction of a push or pull. In this module you will meet four of them: the lever, the wheel and axle, the ramp and the pulley.

The three parts of a lever

A physics textbook from OpenStax describes a lever as a stiff bar that turns on a fixed point, called the fulcrum. Every lever has three parts.

Here is the secret. The handle is long and the tip under the lid is short. Your hand is far from the fulcrum. The lid is close to it. That difference is what turns a small push into a big lift.

Bottom line: A lever turns on a fulcrum. The further your push is from the fulcrum, and the closer the load is, the easier the lift.

Build a ruler lever

You need:

• A 30 centimeter ruler
• A round pencil, for the fulcrum
• Two bottle caps or two small paper cups
• Tape
• About 50 coins, all the same kind

1. Tape the pencil flat to the table, so it cannot roll.
2. Tape one bottle cap near each end of the ruler, open side up. One cap holds the load. The other holds the effort.
3. Put 10 coins in the load cap. This is your load, and it stays the same for every test.
4. Lay the ruler across the pencil with the 15 centimeter mark on the pencil, so the fulcrum is in the middle.
5. Add coins to the effort cap one at a time. Stop when the load end lifts off the table. Count the effort coins.

With the fulcrum in the middle, you will need about as many coins on the effort side as on the load side, a little more to lift it. That is a seesaw: the same distance on each side, so the same weight balances.

Move the fulcrum

Now slide the ruler along so the pencil is under a different mark. Test five places. Change only where the fulcrum is. Keep the same ruler, the same caps and the same 10 load coins. Measure each position from the load end.

Here is a sample of what you might find. Your numbers will be different, partly because the ruler has some weight of its own.

Read the table from top to bottom. When the fulcrum is close to the load, two coins lift ten. When the fulcrum is close to your effort, even forty coins struggle. The numbers change a lot, but nothing else in the test changed. So the fulcrum's position is what made the difference.

Safety: When the load end flips up, coins can jump. Keep your face well away from the ruler, add coins gently, and never slam down on the effort end. Coins are small parts, so keep them away from babies and toddlers.

The trade: less push, more distance

Does a lever give you something for nothing? No. Try this. Put the fulcrum 5 centimeters from the load end. Push the effort end down with your finger and watch both ends against a ruler held upright beside them.

Your end moves a long way down. The load end moves only a little way up. You pushed gently, but you pushed a long way. The load moved a short way, but with a strong lift.

The OpenStax textbook puts it like this: simple machines trade force for distance. A lever can make your push stronger, but only by making you move further. Engineers choose where to put the fulcrum depending on which they need more: a strong lift, or a big movement.

The point: A lever does not make work disappear. It lets you swap a hard, short push for an easy, long one.

Levers all around you

Once you know the three parts, you will see levers everywhere. The fulcrum is not always in the middle.

The OpenStax textbook uses a wheelbarrow as its example. The wheel's axle is the fulcrum, the load sits in the middle, and your hands lift the handles at the far end. That is why you can wheel a load you could never carry.

A shovel is the opposite. Your top hand is the fulcrum, your lower hand gives the effort, and the load is at the far end. You push harder than the weight of the dirt, but the blade moves a long way, fast. Not every lever is for making things easier to lift.

Scissors are two levers joined at the screw in the middle. Cut thick card near the screw, and it is easier than cutting with the tips. Now you know why: near the screw, the card is close to the fulcrum.

Could a lever move the world?

Archimedes was a Greek mathematician who lived more than 2,000 years ago, in the city of Syracuse. About 300 years after he died, a writer called Plutarch told stories about him. In a translation of Plutarch printed in 1917, Archimedes writes to his king, Hiero, "that with any given force it was possible to move any given weight". Then he goes further: "if there were another world, and he could go to it, he could move this."

Was he right? In a way, yes. Your ruler test showed that moving the fulcrum closer to the load lets a small effort lift a big one. With a long enough lever and somewhere to stand, the sums say any weight could be moved. But the lever would have to be unimaginably long, and the end of it would have to move unimaginably far. Real engineers have to build things that fit in the real world.

For the grown-up helping: If your child is ready, show the pattern in the table as a balance rule: load coins times their distance from the fulcrum roughly equals effort coins times their distance. With the fulcrum 10 centimeters from the load and 20 from the effort, about half as many effort coins are needed. The ruler's own weight makes real results drift from the rule.

Common misconceptions

• Mistake: a lever gives you extra energy for free. It lets you push less, but you must push further. The work is traded, not created.
• Mistake: the fulcrum always goes in the middle. A seesaw has it in the middle, but a wheelbarrow, a shovel and a paint can opener do not.
• Mistake: every lever makes lifting easier. A shovel or a broom makes you push harder, in return for moving the far end quickly and a long way.
• Mistake: a bigger child always wins on a seesaw. A lighter child sitting further from the fulcrum can balance, or even lift, a heavier one sitting close to it.

Where this leaves us

• A lever is a stiff bar that turns on a fixed point called the fulcrum.
• Every lever has a fulcrum, an effort and a load.
• Moving the fulcrum closer to the load means less effort is needed.
• The trade: with less push, your end has to move further.
• Levers include seesaws, scissors, wheelbarrows, bottle openers, shovels and tweezers.
• Plutarch's story of Archimedes says he boasted that he could move the world, given somewhere to stand.

Sources

1. Urone, P. P., and Hinrichs, R. (2022). 9.5 Simple machines. In College physics 2e. OpenStax. openstax.org
2. Plutarch. (1917). Marcellus (B. Perrin, Trans.). In Lives (Vol. 5, Loeb Classical Library). Harvard University Press. (Original work written about 100 CE). LacusCurtius, University of Chicago
3. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

A wheel from the marsh

In 2002, archaeologists working in marshy ground about 20 kilometers from Ljubljana, the capital of Slovenia, dug up a wooden wheel. Lying near it was a wooden axle, 120 centimeters long. Tests on the wood showed that the wheel was between 5,100 and 5,350 years old.

Slovenia's government website says it is the oldest wheel ever found anywhere in the world. The City Museum of Ljubljana, which looks after it, describes it as the oldest wooden wheel with an axle. It is made of two boards of ash wood, cut from the same tree, and its axle is made of oak. Somebody, more than 5,000 years ago, chose two different woods for two different jobs. That is engineering.

In this lesson you will follow wheels from that marsh all the way to Mars. Along the way you will build a car of your own.

Wheel, axle and friction

A wheel is a round disk that turns. An axle is the rod that a wheel turns on, or turns with. Together they make one of the simple machines: the wheel and axle.

Why bother with wheels? Try this. Put a heavy book on the table and push it along. It is hard work, and it scrapes. Now lay four round pencils under the book like rollers and push again. The book glides.

The force that makes sliding hard is called friction (say it: FRIK-shun). A physics textbook from OpenStax defines friction as a force that works against things sliding over each other. Rub your hands together fast and they get warm. That warmth is friction at work.

When the book slides, its whole bottom rubs along the table. When it rolls on pencils, the pencils turn instead of rubbing. A wheel does the same job as the pencils, but it stays with the load instead of being left behind.

Why this matters: Wheels let a load roll instead of slide, which cuts down the rubbing that makes moving things hard.

The rub moves to the axle

A wheel does not get rid of friction completely. Something still has to rub: the axle, where it turns. Engineers make that rubbing as small and smooth as they can.

A part that holds a turning axle and lets it spin smoothly is called a bearing. In your car, the bearings will be pieces of drinking straw. The axle turns inside the straw, which is smooth and slippery. Real cars and bikes use bearings with tiny steel balls inside, packed in grease. Grease is a lubricant, a slippery substance that cuts down friction. OpenStax notes that lubrication makes friction much smaller.

A wheel and axle can also make turning easier

Wheels are not only for rolling. Turn a doorknob, and the big knob turns a thin rod inside the door. Turn a steering wheel, and the big wheel turns a thin shaft. The OpenStax physics book calls the wheel and axle a form of lever, and gives exactly these examples.

Here is why it helps. Your hand on the rim of the big wheel is far from the middle, like your hand on the long end of a lever. A small push on the rim makes a strong twist on the thin axle. Try turning the bare metal rod of a doorknob with your fingers after a grown-up takes the knob off. It is much harder.

Build a cardboard car

You need:

• A piece of corrugated cardboard about 10 centimeters by 20 centimeters, cut by a grown-up
• 2 drinking straws
• 2 wooden skewers, with their sharp points cut off by a grown-up
• 4 wheels: plastic bottle caps, or circles of cardboard about 6 centimeters across
• Tape and safety scissors
• A little play dough or modeling clay
• A ramp: a large hardback book or a board, propped on a stack of books 10 centimeters high
• A tape measure

1. Cut each straw so it is a little shorter than the width of the cardboard.
2. Tape the two straws across the bottom of the cardboard, one near each end. Make sure they are straight and exactly parallel. Measure the gap at both ends to check.
3. A grown-up makes a small hole in the middle of each wheel with a sharp pencil or a thumbtack.
4. Slide a skewer through each straw. It should spin freely.
5. Push a wheel onto each end of each skewer. Press a small blob of play dough around the skewer where it meets each wheel, so the wheel turns with the skewer.
6. Spin each axle with your fingers. The wheels should turn freely and not rub on the cardboard.

Safety: Skewer points are sharp, so a grown-up cuts them off before you start. A grown-up makes the holes in the wheels and cuts the cardboard. If you use hot glue instead of play dough, a grown-up does the gluing. Bottle caps are small parts, so keep them away from babies and toddlers.

The ramp test

1. Mark a starting line near the top of the ramp with a strip of tape.
2. Hold the car with its front wheels on the line, and let go. Do not push.
3. When the car stops, measure how far it rolled from the bottom of the ramp, in centimeters.
4. Do three runs and use the middle number.

Here is a sample of what three versions of the car might do. Your numbers will be different.

Version 1 slides. Version 2 rolls, and goes about fifteen times further. Version 3 shows that a small mistake in building matters: crooked bearings make the wheels fight each other and rub. Each version changed one thing from version 2, so you can tell what made each difference.

Wheels on Mars

Wheels can wear out too, and that brings us to Mars. NASA's Curiosity rover landed on Mars on 6 August 2012. It has six aluminum wheels. About 14 months after it started driving, engineers saw an alarming number of cracks appearing in the thin metal skin of its wheels. NASA says the wheels wore down from driving over sharp, pointy rocks.

When NASA built its next rover, Perseverance, engineers used what they had learned. They tested new wheels in a yard full of rocks on Earth before sending them to Mars. Compare the two in the table.

NASA says the new treads better withstand the pressure from sharp rocks, and grip sand as well as or better than the old ones. Each Perseverance wheel also has curved titanium spokes, which NASA says give it springy support. That is the Improve step of the design process, done by real engineers, for wheels that had to work on another planet.

What matters here: Engineers watch how their designs wear out, and use what they see to build the next one better.

Common misconceptions

• Mistake: wheels get rid of friction. Wheels cut down rubbing on the ground, but the axle still rubs where it turns. Bearings and grease make that rubbing small.
• Mistake: friction is always bad. Without friction between the wheels and the ground, the wheels would just spin. Treads on tires and on Mars rover wheels are there to grip.
• Mistake: if the parts are all there, the car will roll well. How carefully it is built matters. Crooked bearings made the sample car roll less than half as far.
• Mistake: the wheel was invented once and has not changed since. The Ljubljana wheel was made more than 5,000 years ago, and engineers were still redesigning wheels for Mars in the 2010s.

The takeaway

• A wheel turns on, or with, a rod called an axle. Together they make the simple machine called the wheel and axle.
• Friction is a force that works against sliding. Rolling on wheels cuts down the rubbing.
• Bearings and lubricants make the rubbing at the axle small and smooth.
• A big wheel turning a thin axle, like a doorknob, makes turning easier.
• In a fair ramp test, release the car from the same mark without pushing, and measure how far it rolls.
• The Ljubljana wheel is about 5,200 years old. NASA redesigned the wheels of Perseverance after Curiosity's wheels cracked on sharp rocks.

Sources

1. Glogovcan Belancic, T. (2020, January 6). World's oldest wheel found in Slovenia. Slovenia.si. Government Communication Office of the Republic of Slovenia. slovenia.si
2. City Museum of Ljubljana. (2013). The wheel: 5,200 years [Exhibition]. mgml.si
3. NASA Science. (n.d.). Perseverance rover components. science.nasa.gov
4. NASA. (2020, April 3). NASA's Perseverance Mars rover gets its wheels and air brakes. nasa.gov
5. Rink, K., Wang, J.-K. F., Graser, E., Patel, N., and Rankin, A. (2022). Assessing Mars Curiosity rover wheel damage. NASA Technical Reports Server. ntrs.nasa.gov
6. Urone, P. P., and Hinrichs, R. (2020). 9.3 Simple machines. In Physics. OpenStax. openstax.org
7. Urone, P. P., and Hinrichs, R. (2022). 5.1 Friction. In College physics 2e. OpenStax. openstax.org

One inch up, twelve inches along

A rule published by the U.S. Department of Justice in 2010 says that a ramp built for wheelchairs may rise no more than 1 inch for every 12 inches it runs along. The rule is part of the 2010 ADA Standards for Accessible Design. ADA stands for the Americans with Disabilities Act.

So a step 6 inches high needs a ramp at least 72 inches long. That is 6 feet, about the height of a tall grown-up, lying down. The same rules say a single run of ramp may rise no more than 30 inches, and it needs a flat landing at the top and the bottom.

That makes ramps long. Rising 30 inches at 1 in 12 takes 360 inches of ramp, which is 30 feet. Why would anyone build something so long to climb so little? That question starts an argument, and this lesson settles it with measurements.

The argument

A ramp is an inclined plane: a flat surface with one end higher than the other. An OpenStax physics book says that every sloping path or road for walking or driving is an inclined plane.

Imagine two friends arguing about ramps.

Maya says: "A ramp makes lifting easier. Pulling a cart up a ramp hardly takes any effort. Lifting it straight up is hard."

Leo says: "A ramp does not save anything. You have to go so much further. Lifting straight up is quicker."

Both of them sound reasonable. Each one is pointing at something real. An engineer would not pick a side by arguing. An engineer would measure. Maya is talking about how hard you pull. Leo is talking about how far you pull. So you need to measure both.

Build a pull meter

You will measure pulls with a rubber band. The harder you pull on a rubber band, the further it stretches. So the length of the band tells you how hard you are pulling.

You need:

• A rubber band, about 8 centimeters long when it is not stretched
• A paper clip
• A toy car or truck with wheels that roll freely
• 20 coins and some tape
• A stiff, smooth board about 60 centimeters long, such as a shelf or a piece of thick cardboard
• Books to make stacks 5, 10, 15 and 20 centimeters high
• A ruler

1. Tape the 20 coins onto the toy car. This is your load, and it stays the same for every test.
2. Open the paper clip into a hook. Hook one end onto the front of the car and loop the rubber band through the other end.
3. Measure the rubber band before you pull. Write down its length.

Safety: Rubber bands can snap. Hold the band away from your face, never stretch it toward anyone, and wear safety glasses if you have them. Build the book stacks low and steady. Keep coins away from babies and toddlers.

Maya's evidence: the pull

1. Straight up. Hold the end of the rubber band and lift the car slowly straight up off the table. While it hangs still, a helper measures the stretched band with the ruler.
2. Up the ramp. Rest one end of the board on a stack of books 20 centimeters high. Put the car at the bottom. Pull it slowly and steadily up the ramp by the rubber band, keeping the band parallel to the board. Halfway up, your helper measures the band.
3. Do the same with stacks 15, 10 and 5 centimeters high.

Subtract the band's starting length from each measurement. What is left is the stretch. More stretch means a harder pull.

Here is a sample of what you might find. Your numbers will be different.

Maya is right about the pull. Lifting straight up stretched the band 12 centimeters. Pulling up the ramp to the same height stretched it only 5. And the gentler the ramp, the smaller the pull.

Leo's evidence: the distance

Now look at the distance column. To raise the car 20 centimeters by lifting, your hand moved 20 centimeters. To raise it the same 20 centimeters with the ramp, your hand moved the whole length of the board: 60 centimeters. That is three times as far.

Leo is right too. The ramp made you go much further.

What settles it

Put the two columns side by side. The ramp cut the pull to well under half. It also made the distance three times as long. You did not get something for nothing. You swapped a big pull over a short distance for a small pull over a long distance.

The OpenStax textbook says exactly this. Pushing a cart up a ramp takes less force than lifting it straight up, but if the rubbing called friction is small, the work done is the same either way. Another OpenStax book puts the trade in plain words: force can decrease if distance increases.

Notice one more thing in the sample. The ramp's pull of 5 is a little more than one third of 12, even though the ramp is three times as long as the lift. That small extra is friction, in the wheels and axles of the car. Ramps waste a little effort. The wheels you met in Lesson 10 keep that waste small.

The upshot: A ramp does not make the job smaller. It spreads the job out, so you can do it with a smaller push.

Now the ADA rule makes more sense. A person wheeling themselves up a ramp has to push the whole time. A gentler slope means a smaller push at every moment. The price is a longer ramp.

For the grown-up helping: Rubber bands do not stretch in exact proportion to the pull, especially near their limits, so treat the stretch as a ranking of pulls rather than a precise measurement. The pattern, smaller pulls on gentler ramps and longer distances, is what the test is built to show.

Ramps in disguise

Once you know the trade, you will find inclined planes everywhere.

• Roads up mountains often zigzag back and forth instead of going straight up. The zigzag makes the road longer and the slope gentler, so cars and cyclists need a smaller push.
• Wedges. OpenStax describes a wedge as simply two inclined planes back to back. A doorstop and the head of an axe are wedges.
• Screws. Look closely at a screw and you will see a ramp spiraling around it, called the thread. OpenStax calls a screw a lever attached to a circular inclined plane. Turning the screwdriver round and round, a long way, drives the screw a short way into the wood with a strong push.

Worth holding on to: Ramps, wedges and screws all make the same trade as levers: a smaller push, over a longer distance.

Common misconceptions

• Mistake: a ramp makes the load lighter. The car weighs the same on the ramp. Only the pull you need is smaller, because part of the car's weight is held up by the ramp itself.
• Mistake: a steeper ramp is a better ramp. A steeper ramp is shorter, but it needs a bigger pull. Which is better depends on the criteria: room to build, or ease of pushing.
• Mistake: ramps are only for wheelchairs. Strollers, delivery carts, wheeled suitcases, bikes and cars all use ramps and sloping roads.
• Mistake: a simple machine gives you extra energy. Every simple machine in this module trades force for distance. None of them makes work disappear.

Pulling it together

• An inclined plane is a flat surface with one end higher than the other, such as a ramp or a sloping road.
• The 2010 ADA Standards say a wheelchair ramp may rise no more than 1 inch for every 12 inches along.
• A rubber band pull meter shows that the gentler the ramp, the smaller the pull.
• The ramp makes you go further. Maya and Leo were both right, about different things.
• The trade: a smaller force over a longer distance. Friction wastes a little extra.
• Wedges and screws are inclined planes too.

Sources

1. U.S. Department of Justice. (2010, September 15). 2010 ADA standards for accessible design (Sections 405.2, 405.6 and 405.7). ada.gov
2. Urone, P. P., and Hinrichs, R. (2022). 9.5 Simple machines. In College physics 2e. OpenStax. openstax.org
3. Urone, P. P., and Hinrichs, R. (2020). 9.3 Simple machines. In Physics. OpenStax. openstax.org

The wheel at the top of the flagpole

Look at the top of a flagpole. There is a small wheel there, with a groove around its edge. A rope runs up the pole, over the wheel, and back down. Pull the rope down, and the flag goes up.

That little wheel is a pulley: a wheel with a groove around its rim for a rope or string to run in. It is the last of the four simple machines in this module.

Does the pulley make the flag lighter? Does it make lifting easier? You are going to build pulleys and measure what they really do. This lesson walks you through one build from start to finish, then changes one thing and watches the result flip.

What you need

• 2 empty thread spools, also called cotton reels
• 2 pencils that fit through the holes in the spools
• About 2 meters of string
• Tape
• 1 paper cup with a bucket handle made from 3 paper clips, as in Lesson 2
• 1 extra paper clip
• 20 coins, all the same kind
• The rubber band pull meter from Lesson 11, and a ruler
• A table, and a folded towel for the floor

Safety: Put the towel on the floor under the cup, in case coins fall, and keep your feet out from underneath. Never put string around anyone's neck. Keep fingers out of the way of a spinning spool. Hold the rubber band away from your face. Keep coins away from babies and toddlers.

Worked example 1: a fixed pulley

A fixed pulley stays in one place. The flagpole pulley is a fixed pulley.

1. Push a pencil through one spool. The spool should spin easily on the pencil.
2. Tape both ends of the pencil firmly to the top of the table, so the spool sticks out just past the edge and can spin freely.
3. Put 20 coins in the cup. This load stays the same for every test.
4. Tie one end of the string to the cup's handle. Let the cup hang below the table edge, run the string up over the spool, and lay the rest across the table top.
5. Tie the rubber band to the free end of the string.
6. Pull the rubber band slowly and steadily along the table, away from the edge. The cup rises.

You pulled sideways, across the table, and the cup went up. That is the first thing a pulley does: it changes the direction of a pull.

Now measure. First, lift the cup straight up by hand with the rubber band, and measure the stretch. Then pull it up with the pulley and measure the stretch while it is rising. Then mark the string with a pen at the table edge, pull until the cup has risen 10 centimeters, and measure how far the mark moved.

Here is a sample. Your numbers will be different.

The fixed pulley did not make the pull smaller. It made it a little bigger, because the spool rubs on the pencil as it turns. That rubbing is friction, which you met in Lesson 10.

The OpenStax physics textbook says the same thing: an ordinary pulley only changes the direction of the force, not its size. So why use one? Because a sideways or downward pull is often much easier to give. At the flagpole, you stay on the ground and pull down, and the flag goes all the way to the top.

Remember: A single fixed pulley changes the direction of your pull. It does not make the pull smaller.

Worked example 2: a movable pulley

Now change one thing. Instead of the cup hanging from the end of the string, the cup will hang from a second pulley that rides on the string. This is a movable pulley, because it moves up with the load.

1. Push the second pencil through the second spool.
2. Open the extra paper clip into a hook. Hang the cup's handle from it, and hook it around the middle of the pencil, next to the spool, so the cup hangs from the second spool's pencil.
3. Untie the string from the cup. Tie that end of the string to a heavy table leg, or tape it very firmly to the table edge, a little way along from the first spool.
4. Run the string down from where it is tied, under the second spool, back up over the first spool, and across the table to your rubber band.
5. Pull slowly and steadily, and measure again.

Now count the strands of string holding up the cup. There are two: one going up to the tied end, and one going up to the first spool. The load is shared between them.

The result flipped. With the movable pulley, the pull dropped to a little over half. But to raise the cup 10 centimeters, your hand had to move 20.

It is the same trade you found with levers and ramps: a smaller pull, over a longer distance. The pull is a little more than half, because the second spool and pencil have weight too, and both spools rub.

In short: Each extra strand of string holding the load shares the pull, but your hand must move further to lift it.

More strands, smaller pull

Engineers join several pulleys together to share a heavy load between many strands. The OpenStax textbook explains that, if the pulleys were free of friction, the number of strands pulling up on the load is roughly how many times smaller the pull becomes. Two strands, about half the pull. Four strands, about a quarter, but four times the length of rope to pull.

A set of pulleys joined together like this is sometimes called a block and tackle. You can see them hanging from the hooks of building cranes and on sailing boats.

People have known this for a very long time. Plutarch, writing about 300 years after Archimedes, told a story in which Archimedes sat at a distance and, "quietly setting in motion with his hand a system of compound pulleys", drew a loaded ship towards him "smoothly and evenly". No one can check today whether it happened just like that. But compound pulleys really can let a small pull move a very big load, slowly, as long as you pull a great deal of rope.

Pulleys around you

For the grown-up helping: If the spools stick, rub a little pencil graphite or a drop of cooking oil on the pencil where the spool turns, and point out that this is a lubricant. If the tied end of the string slips on the movable pulley test, tie it to a table leg rather than taping it.

Common misconceptions

• Mistake: any pulley makes lifting easier. A single fixed pulley only changes the direction of the pull. It needs the same pull, or a little more because of friction.
• Mistake: a movable pulley gives you something for nothing. It roughly halves the pull, but you must pull twice as much string.
• Mistake: more pulleys are always better. More pulleys mean a smaller pull, but much more rope to pull and more rubbing at every wheel.
• Mistake: the pulley wheel carries the load. The strands of rope carry the load. Count the strands holding the load up, and you have a rough idea how much the pull is shared.

Looking back

• A pulley is a wheel with a groove for a rope or string.
• A fixed pulley stays in place. It changes the direction of the pull but not its size.
• A movable pulley rides up with the load. Two strands share the load, so the pull is about half, but your hand moves twice as far.
• The more strands hold the load, the smaller the pull and the longer the rope you must pull.
• Friction at each wheel makes the real pull a little bigger than the ideal one.
• Flagpoles, blinds, wells and cranes all use pulleys.

Sources

1. Urone, P. P., and Hinrichs, R. (2022). 9.5 Simple machines. In College physics 2e. OpenStax. openstax.org
2. Urone, P. P., and Hinrichs, R. (2020). 9.3 Simple machines. In Physics. OpenStax. openstax.org
3. Plutarch. (1917). Marcellus (B. Perrin, Trans.). In Lives (Vol. 5, Loeb Classical Library). Harvard University Press. (Original work written about 100 CE). LacusCurtius, University of Chicago

Same foil, two different endings

Take two squares of kitchen foil, each 10 centimeters by 10 centimeters. Scrunch the first one into a tight ball and drop it into a bowl of water. It sinks to the bottom.

Fold up the edges of the second square to make a little open box, like a tray, and set it on the water. It floats. Gently add coins, one at a time. It keeps floating, and it carries twenty or more.

The two pieces of foil weigh exactly the same. Neither is made of a lighter material. So why does one sink and the other float, and even carry a load? This lesson answers that question, and then you will use the answer to design a better boat.

Pushing water out of the way

Fill a clear glass halfway with water and put a strip of tape on the outside to mark the water level. Now push a toy or an apple down into the water. The water level rises above the tape.

The toy has pushed some water out of the way to make room for itself. Pushing water aside like this is called displacing it (say it: dis-PLACE-ing). The water that was pushed aside had to go somewhere, so the level rose.

Now feel what the water does back. Push an empty plastic bottle, with its cap on, down into a sink of water. The water pushes it back up, hard. The deeper you push, the harder the water pushes. That upward push from the water is called buoyancy (say it: BOY-un-see).

Archimedes' principle

More than 2,000 years ago, the Greek mathematician Archimedes, whom you met in Lessons 9 and 12, worked out how big that upward push is. His rule is called Archimedes' principle, and a physics textbook from OpenStax states it like this: the buoyant force on an object equals the weight of the fluid it displaces.

In kid words: the water pushes up on a thing as hard as the weight of the water that thing pushes aside.

That gives a simple test for floating.

• If a thing can push aside its own weight of water before it is completely under, it floats.
• If it goes completely under and still has not pushed aside its own weight of water, it sinks.

Key idea: Whether a thing floats depends on how much water it pushes aside, compared with how heavy it is.

Back to the foil

Now the puzzle comes apart. The foil ball is tiny. Even when it is completely under the water, it pushes aside only a few drops. A few drops of water weigh less than the foil, so the upward push is too small, and the ball sinks.

The foil boat takes up lots of room, because it is mostly air inside its walls. As it settles into the water, it pushes aside a lot of water. Long before the water reaches the top of its walls, it has pushed aside enough water to match its weight. So it floats.

Add a coin, and the boat sinks a little lower, pushing aside a little more water to match the extra weight. Add more and more coins, and the boat sinks lower and lower. At last the water reaches the top of the walls and pours in. Now the boat cannot push aside any more water, and down it goes.

The OpenStax textbook says a boat shape pushes aside more water than a lump of the same material, which gives it a bigger upward push, and adds: the same is true of steel ships. A giant steel ship floats for exactly the same reason as your foil boat. Its hull is a huge shell with a lot of air inside.

Light for its size, heavy for its size

Here is another way to say the same thing. Things that are heavy for their size sink, and things that are light for their size float. A pebble is heavy for its size. A cork is light for its size.

The clever part is what counts as the boat's size. The size includes all the air inside its walls. Foil on its own is heavy for its size, so the ball sinks. Foil plus a boatful of air is light for its size, so the boat floats. Engineers who design ships use this all the time.

The story of the bath

There is a famous story about Archimedes. He was in a public bath, the story goes, and noticed the water rising as he sat down. He was so excited by the idea that he jumped out and ran through the streets without his clothes, shouting "Eureka!", which is Greek for "I have found it!"

Is it true? Nobody knows. The OpenStax textbook says he "purportedly" did this, which means people say so, but it has not been proven. It is a good story, but treat it as a story. Archimedes' principle, on the other hand, you can test for yourself today.

Build and test foil boats

You need:

• Kitchen foil, cut into squares 10 centimeters by 10 centimeters
• A ruler and a pencil
• A large bowl, a sink, or a baking tray, with water about 5 centimeters deep
• About 50 coins, all the same kind
• A towel for spills

Your fair test: does the height of the walls change how many coins a boat can carry? Use the same size of foil every time, so only the wall height changes.

1. On a foil square, draw a line 1 centimeter in from each edge. Fold up the edges along the lines to make walls 1 centimeter high. Pinch the corners so they do not leak.
2. Make a second boat with walls 2 centimeters high, and a third with walls 3 centimeters high.
3. Float the first boat. Add coins one at a time, spreading them out evenly over the bottom.
4. When water pours in and the boat sinks, stop. The result is the number of coins it held before the last one.
5. Dry the coins and test the other boats the same way. Test each boat three times, with a fresh square each time, and use the middle number.

Here is a sample. Your numbers will be different.

Look at what happened. Higher walls did not always win. With the same square of foil, higher walls leave a smaller floor. The 1 centimeter boat had a big floor but low walls, so water soon spilled in. The 3 centimeter boat had tall walls but a tiny floor. The middle boat had the most room for water to be pushed aside, so it carried the most.

So what? A boat carries the most when it can push aside the most water before its walls go under. Engineers balance the floor and the walls to get the most room from the material they have.

For the grown-up helping: One cubic centimeter of water weighs about one gram, so a boat can hold roughly as many grams as the number of cubic centimeters inside it, minus its own weight. The 2 centimeter boat holds 6 times 6 times 2, which is 72 cubic centimeters, so about 72 grams. That is why the middle wall height wins for a 10 centimeter square.

Water safety

Water experiments are fun, and they need a few firm rules.

• A grown-up is always nearby when there is water out. The U.S. Centers for Disease Control and Prevention warns that drowning happens quickly and quietly, and that a responsible adult should watch children closely and constantly whenever they are in or near water, including bathtubs.
• Use a bowl, a tray or a sink, not a full bathtub, and empty it as soon as you finish.
• Keep babies and toddlers away from the water and the coins.
• Wipe up spills straight away so nobody slips, and keep water well away from anything that plugs in.

Common misconceptions

• Mistake: heavy things sink and light things float. A steel ship weighs thousands of tonnes and floats. What matters is weight compared with the water it pushes aside, or how heavy it is for its size.
• Mistake: things float because air holds them up. The water does the pushing. The air inside a boat matters because it makes the boat big for its weight, so it pushes aside a lot of water.
• Mistake: taller walls always carry more. With the same amount of foil, taller walls mean a smaller floor. The best boat balances the two.
• Mistake: the Eureka story is a proven fact. It is an old story that may or may not be true. The principle is what you can test.

The short version

• Anything in water pushes some water aside. This is called displacing it.
• The water pushes back up. This push is called buoyancy.
• Archimedes' principle: the upward push equals the weight of the water pushed aside.
• A boat shape floats because it pushes aside a lot of water. A ball of the same material sinks.
• With the same square of foil, the best boat balances floor size and wall height.
• A grown-up always supervises water play, because drowning happens quickly and quietly.

Sources

1. Urone, P. P., and Hinrichs, R. (2022). 11.7 Archimedes' principle. In College physics 2e. OpenStax. openstax.org
2. Centers for Disease Control and Prevention. (2025, September 29). Preventing drowning. cdc.gov
3. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

Twelve seconds over the sand

At 10:35 in the morning on 17 December 1903, on a beach at Kitty Hawk in North Carolina, a flying machine lifted off the ground with Orville Wright on board. It flew for 12 seconds and traveled 120 feet, about 36 meters. That is less than the length of a basketball court and back.

It does not sound like much. But the Smithsonian's National Air and Space Museum, which keeps the machine today, calls these the world's first successful flights of a powered airplane with a pilot on board. The brothers flew four times that day, taking turns. The longest flight, by Wilbur, went 852 feet in 59 seconds.

In Lesson 3 you saw how the Wright brothers tested about 200 wing shapes in their wind tunnel in 1901. This lesson follows them to that beach, and uses their machine to explain how anything flies, including the paper plane you are about to fold.

What the brothers built

The machine was called the Wright Flyer. Its wings stretched 12.3 meters from tip to tip. Here is what each part did.

• The wings held the machine up as it moved through the air.
• The engine was small, with twelve horsepower. The brothers built it with help from Charles Taylor, the mechanic in their bicycle shop.
• Two propellers at the back were turned by chains, rather like the chain on a bicycle. The museum says the brothers thought of the propellers as wings that spin.
• Lines worked by the pilot could twist the tips of the wings in opposite directions. This wing warping let the pilot tip the machine to one side or the other, to steer and to keep it level.

Every one of those parts deals with one of four forces. Once you know the four, you can understand any flying thing.

The four forces

NASA's Glenn Research Center teaches that four forces act on an airplane in flight.

You can feel drag yourself. Hold a big, flat piece of cardboard in front of you and walk quickly across a room or a yard. The air pushes the cardboard back toward you. That push is drag. Walk faster, and the push gets stronger.

NASA explains that when the forces are balanced, a plane flies along at a steady speed. When they are not balanced, the plane speeds up in the direction of the biggest force. More lift than weight, and it climbs. More thrust than drag, and it speeds up.

The point: A plane flies when its wings make enough lift to hold up its weight, and something pushes it forward hard enough to beat the drag.

A paper plane is a glider

A paper plane has no engine. NASA calls an aircraft with no engine a glider. Once it leaves your hand, only three forces act on it: lift, weight and drag.

So where does the thrust come from? NASA's guide for teachers on gliders puts it simply: when you throw a paper airplane, you give it thrust, a forward force. After that, nothing pushes it forward. Drag slows it down, and it glides gently toward the floor. NASA says a glider trades height for speed. It keeps moving forward by slowly coming down.

That is why a good paper plane is not the one you throw hardest. It is the one that loses height most slowly while it moves forward.

Fold a paper dart

You need a sheet of printer paper, a ruler, a tape measure, a paper clip, and a long, clear space indoors, where there is no wind.

1. Fold the sheet in half the long way. Crease it hard, then open it out flat.
2. Fold the two top corners in so they meet at the middle crease. You now have a pointed top.
3. Fold the two new slanted edges in to meet the middle crease again. The point gets sharper.
4. Fold the whole sheet in half along the middle crease, with the folds on the outside.
5. Fold one side down to make a wing: line up the slanted top edge with the bottom edge of the body. Crease hard. Turn it over and do the same for the other wing.
6. Open the wings so they stick out flat and level. Seen from the back, the plane looks like the letter T.

Safety: A paper plane's nose is pointed and can hurt an eye. Never throw a plane toward a person or a pet. Throw only in a clear space, and look before you throw. NASA's own glider challenge asks students to wear safety goggles, so wear safety glasses if you have them. Do not climb on furniture to launch or to fetch a plane.

The fair test: does a paper clip help?

Here is a question to test: does putting a paper clip on the nose make the plane fly further?

1. Put a strip of tape on the floor as a throwing line.
2. Stand behind the line and throw the plane gently forward, level with your shoulder.
3. Measure from the line to the spot where the plane first touches the floor, in meters and centimeters.
4. Do five throws without the clip.
5. Slide one paper clip onto the nose, underneath. Change nothing else. Do five more throws.

Keep everything else the same: the same plane, the same thrower, the same line, the same gentle throw, the same room. The only change is the clip.

With five throws, put the distances in order and pick the middle one, the third. Here is a sample. Your numbers will be different.

In the sample, the clip helped. The extra weight at the front moved the plane's balance point forward, and this plane flew steadier. Your plane might behave differently, and too much weight at the front can make a plane dive. That is exactly why you test instead of guessing. Try two clips and see.

For the grown-up helping: Throwing is the hardest thing to keep the same. One person should do all the throws, and practice a few times first. Five throws with a middle number smooths out most of the wobble.

Steer it like the Wrights

The Wright brothers twisted their wing tips to steer. You can try something similar. Bend up the back edges of both wings, just a few millimeters, and throw again. Then bend one up and the other down. Watch what happens. Change one bend at a time, and write down what each change does.

The core of it: Every flying machine, from the Wright Flyer to a paper dart, is a balance of lift, weight, thrust and drag. Engineers test one change at a time to learn how to shift that balance.

Common misconceptions

• Mistake: a paper plane keeps pushing itself forward. It has no engine. All its thrust comes from your throw, and after that it glides, slowly coming down.
• Mistake: throwing harder always makes it go further. A very hard throw often makes the plane swoop up, slow down and dive. A smooth, gentle throw usually glides further.
• Mistake: planes stay up because they are light. Big airliners are very heavy. They stay up because their wings make lift as they move through the air.
• Mistake: the first airplane flight was long and high. The first flight on 17 December 1903 lasted 12 seconds and covered 120 feet. Big things often start small.

What to carry forward

• On 17 December 1903, Orville Wright flew 120 feet in 12 seconds at Kitty Hawk. The longest flight that day, by Wilbur, lasted 59 seconds.
• Four forces act on an airplane: lift up, weight down, thrust forward and drag backward.
• Wings make lift. Engines and propellers make thrust. Air resistance makes drag. Gravity makes weight.
• A paper plane is a glider. Its only thrust comes from your throw.
• A fair paper plane test keeps the plane, the thrower and the throw the same, and changes one thing.
• With five throws, the middle number is the third one when they are put in order.

Sources

1. Smithsonian National Air and Space Museum. (n.d.). 1903 Wright Flyer. Collection objects. airandspace.si.edu
2. NASA Glenn Research Center. (2022, July 21). Four forces on an airplane. Beginner's guide to aeronautics. nasa.gov
3. NASA Glenn Research Center. (2024, July 19). Three forces on a glider. Beginner's guide to aeronautics. nasa.gov
4. NASA Glenn Research Center, Office of Education. (n.d.). Let it glide: Engineering design challenge facilitation guide. nasa.gov

The biggest parachute on Mars

On 18 February 2021, NASA's Perseverance rover dropped through the sky of Mars toward the ground. Above it, a parachute opened in less than a second. Fully open, it was 70.5 feet wide, about 21.5 meters. NASA says it was the largest parachute ever sent to Mars.

A parachute is a simple idea. But why does it work? Most people already have a rule in their heads about falling things: heavy things fall fast, and light things fall slowly. It sounds right. In this lesson you will test that rule, find exactly where it goes wrong, and use what you find to build a better parachute.

Test the rule: two sheets of paper

Take two sheets of printer paper that are exactly the same. They weigh the same. Scrunch one into a tight ball. Leave the other one flat.

Hold them both up high, at the same height, the flat one held level. Let go at the same moment. Which lands first?

The ball drops straight down. The flat sheet drifts, swoops and floats down much later. But they weigh exactly the same. If heavy things fall fast and light things fall slowly, they should have landed together.

So the rule has failed. Something other than weight changed how fast they fell. The only difference between the two sheets is their shape.

Where the rule goes wrong: the air

As something falls, it has to push through the air. The air pushes back. You met this push in Lesson 14: it is called drag, or air resistance.

The flat sheet has a big surface pushing against the air, so the air pushes back on it a lot. The ball has a small surface, so the air pushes back on it only a little. NASA's Glenn Research Center explains that the amount of drag depends on the size of the object. Doubling the area doubles the drag.

So the real rule is not about weight alone. How fast something falls through air depends on its weight and on how much the air pushes back on it.

A hammer and a feather on the Moon

What would happen with no air at all? In 1971, near the end of the Apollo 15 mission, astronaut David Scott stood on the Moon, where there is no air, and did an experiment for the television cameras. He held up a hammer in one hand and a falcon feather in the other, and let go of both at once.

They hit the ground at the same time. NASA explains that they were in a vacuum, a place with no air, so there was no air resistance to slow the feather down. Scott said on the broadcast that this proved Galileo, an Italian scientist from the 1600s, had been right.

Bottom line: The rule that heavy things fall faster fails because of air. Without air, everything falls together. With air, drag slows down things with a big surface for their weight.

Why a falling thing reaches a top speed

Drop something, and at first it speeds up. But the faster it goes, the harder the air pushes back. NASA explains that the drag on a falling object grows as it speeds up, until the drag is exactly equal to the weight. From then on, it stops speeding up and falls at a steady speed.

A parachute makes that happen at a slow speed. Its huge canopy has so much surface that the drag catches up with the weight very soon, while the load is still moving slowly. Then it drifts down gently.

Mars made this hard for Perseverance. NASA says the air on Mars is very thin, with only about one percent of the pressure of the air on Earth. Thin air pushes back much less, so the parachute had to be enormous. Even then, it could not do the whole job. The spacecraft also used rockets to slow down before it touched the ground.

Build a parachute

You need:

• Tissue paper or paper napkins, cut into squares 15, 25 and 35 centimeters across
• String, cut into pieces 30 centimeters long, four for each parachute
• Tape and safety scissors
• Paper clips, and 2 coins for the load
• A stopwatch or a phone timer, worked by a grown-up

1. Tape one string to each corner of a canopy square.
2. Hold the canopy up by its middle and gather the four loose ends of string together, all the same length.
3. Tie the ends to a paper clip. Tape the 2 coins to the paper clip. This load stays the same for every parachute.

Safety: Do not use plastic bags for parachutes, and keep plastic bags away from babies and toddlers. Do not climb on chairs, tables or railings to drop things. A grown-up does the drops, standing on the floor. Keep coins and paper clips away from younger brothers and sisters.

The canopy test

1. A grown-up stands on the floor and holds the parachute up high, at the same height every time, about 2 meters, with the canopy spread out.
2. On "go", they let go and you start the timer. Stop it when the coins touch the floor.
3. Drop each parachute three times and use the middle time.
4. Also time the load on its own, with no canopy at all.

Change only the size of the canopy. Keep the material, the strings, the load and the height the same. Here is a sample. Your times will be different.

The bigger the canopy, the more drag, and the slower the fall, just as NASA's drag pages predict. Watch for failure points too. A big canopy may tip, swing or tangle its strings. That gives you something to improve: a small hole cut in the middle of the canopy, or shorter strings.

A paper spinner, and a helicopter on Mars

Parachutes slow a fall with drag. A spinner does something cleverer. On 19 April 2021, NASA's Ingenuity helicopter lifted off from the ground of Mars. NASA calls it the first aircraft in history to make a powered, controlled flight on another planet. It rose 3 meters, hovered, and landed, after 39.1 seconds in the air. It weighed about 1.8 kilograms, and its rotor blades measured 1.2 meters from tip to tip.

NASA's Jet Propulsion Laboratory has a paper helicopter activity for students in grades 2 to 8. Its rotor blades are angled, so they push against the air as they turn. A paper spinner has no motor, so it cannot climb, but as it falls its angled blades make it spin, and it comes down slowly. Here is one way to make one.

1. Cut a strip of paper 21 centimeters long and 5 centimeters wide.
2. From the top, cut straight down the middle for 9 centimeters. These two flaps are the blades.
3. About 2 centimeters below the bottom of that cut, make a small cut 1.5 centimeters in from each side.
4. Fold the two side pieces below those cuts in toward the middle, one over the other. This is the body.
5. Fold the bottom 2 centimeters up, and hold it with a paper clip.
6. Fold one blade forward and the other backward.

Hold it by the body, as high as you can, and let go. It spins. Now test it fairly: make spinners with blades 9, 6 and 4 centimeters long, keep everything else the same, and time three drops of each from the same height.

What matters here: Air can be used to slow things down. Engineers choose the size and shape of a canopy or a blade to get the fall, or the flight, they need.

For the grown-up helping: Hand timing of drops under three seconds is rough, so three drops and a middle time are important. If you have a phone, filming the drops and counting frames gives a more exact time, and is a good introduction to measuring tools.

Common misconceptions

• Mistake: heavier things always fall faster. Two sheets of the same weight fell at different speeds because of their shape. On the Moon, with no air, a hammer and a feather landed together.
• Mistake: a parachute stops the fall. A parachute slows the fall to a gentle, steady speed. The load still comes down.
• Mistake: a bigger parachute is always better. A bigger canopy gives more drag, but it can also tip, swing or tangle. Engineers test for those failure points too.
• Mistake: air is nothing. Air is made of real stuff that pushes back. You can feel it when you run holding a big sheet of card.

Recap

• Perseverance landed on Mars on 18 February 2021 with a parachute 21.5 meters wide, the largest ever sent to Mars.
• The rule that heavy things fall faster fails because of air resistance, or drag.
• Drag depends on the size of an object. Doubling the area doubles the drag.
• A falling object speeds up until drag equals its weight, then falls at a steady speed.
• On the Moon in 1971, with no air, a hammer and a feather landed at the same time.
• In a fair canopy test, bigger canopies fall more slowly. Ingenuity made the first powered, controlled flight on another planet in 2021.

Sources

1. NASA Jet Propulsion Laboratory. (2021, February 22). NASA's Mars Perseverance rover provides front-row seat to landing, first audio recording of Red Planet. jpl.nasa.gov
2. NASA. (2021, April 19). NASA's Ingenuity Mars helicopter succeeds in historic first flight. nasa.gov
3. NASA Jet Propulsion Laboratory. (2025). Make a paper Mars helicopter [Lesson plan]. jpl.nasa.gov
4. NASA Glenn Research Center. (2024, June 21). Size effects on drag. Beginner's guide to aeronautics. nasa.gov
5. NASA Glenn Research Center. (2024, July 18). Falling object with air resistance. Beginner's guide to aeronautics. nasa.gov
6. NASA Science. (2018, July 16). The Apollo 15 hammer-feather drop. science.nasa.gov

A bronze ball in a wooden groove

In 1638, the Italian scientist Galileo Galilei published a book that described a careful experiment with a ramp. In an English translation printed in 1914, he describes taking "a piece of wooden moulding or scantling, about 12 cubits long". A cubit was an old measure, roughly the length of a grown-up's forearm, so this board was several meters long.

Along its edge he cut a groove a little wider than a finger. He made it "very straight, smooth, and polished" and lined it with parchment, a smooth material made from animal skin. Then, he writes, "we rolled along it a hard, smooth, and very round bronze ball". One end of the board was lifted one or two cubits higher than the other, so the ball rolled downhill.

Galileo had no stopwatch. So he used water. Water ran out of a thin pipe at the bottom of a big container. While the ball rolled, he caught the water in a glass. Then he weighed the water. More water meant more time. He repeated the experiment, he says, "a full hundred times".

A marble run is Galileo's ramp turned into a game. In this lesson you will find out what happens to a marble as it rolls down, test it fairly, and then build a run of your own.

Height is stored energy

Energy is what makes things move, light up, warm up or make a sound. A marble sitting at the top of a ramp is not moving yet, but it has energy stored in it, just because it is high up. Scientists call this stored energy, or potential energy.

Let go, and the marble rolls. As it goes down, its stored energy turns into the energy of movement. The lower it gets, the faster it goes.

NASA describes the same trade for gliders: a glider trades height for speed. A marble on a ramp does exactly that.

The science standards for fourth grade say that the faster an object is moving, the more energy it has. They also say that when objects bump into each other, energy can pass from one to the other and change how they move. You are about to test both ideas with a paper cup.

Worth holding on to: A marble's height is stored energy. Rolling down turns that height into speed.

The cup test

How can you tell how much energy a marble has at the bottom of a ramp? Let it hit something, and see how far it pushes it.

You need:

• A marble
• A ramp: a piece of card folded lengthwise into a V-shaped channel, or a paper towel tube cut in half lengthwise by a grown-up, about 50 centimeters long
• Books to prop up the top of the ramp
• A small paper cup with a doorway cut in its rim, laid on its side at the bottom of the ramp so the marble rolls into it
• A ruler and a pencil
• A smooth, flat floor or a long table

1. Prop the ramp on the books so its top end is about 20 centimeters high.
2. Mark four starting points on the ramp with a pencil: 5, 10, 15 and 20 centimeters above the floor. Measure straight up from the floor to each mark.
3. Lay the cup on its side just past the bottom of the ramp, with its doorway facing the ramp. Draw a line on the floor along the back of the cup.
4. Hold the marble at the 5 centimeter mark and let go. Do not push.
5. Measure how far the cup slid from its line, in centimeters.
6. Put the cup back on its line. Do three tries from each mark and use the middle number.

Change only the starting height. Keep the marble, the ramp, the cup and the floor the same. Here is a sample. Your numbers will be different.

The higher the start, the further the cup slid. A higher start gave the marble more stored energy, so it was moving faster at the bottom. A faster marble has more energy to pass to the cup when they bump.

Why the first hill is the highest

Bend a long strip of card into a U shape, like a skateboard ramp. Let a marble go from the top of one side. It rolls down, races through the bottom, and climbs up the other side. But it never quite reaches the height it started from. Then it rolls back, and climbs a little less, and a little less, until it stops.

Where did the energy go? Some was used up by rubbing on the card, which you met as friction in Lesson 10. Some went into pushing the air and into the clicking sound the marble makes. None of it disappeared, but it is no longer helping the marble move.

That is why the first hill of a roller coaster, and the start of a marble run, has to be the highest point. A marble cannot climb higher than where it began. Every later hill must be lower.

Why this matters: When you design a marble run, you spend the height you start with. Every drop gives speed, and every bit of rubbing takes a little away.

Design a marble run

Now build a whole run. Use the side of a big cardboard box laid flat against a wall, or the box itself.

Your criteria: the marble must start at the top by itself, with no push, and finish in a cup at the bottom. It must go round at least one turn and over at least one drop. Your constraints: one cardboard box, 6 paper towel or toilet paper tubes, card, and 2 meters of tape.

Useful parts:

• Channels. Tubes cut in half lengthwise by a grown-up, or card folded into a V.
• Tunnels. Whole tubes, taped on a slope. The marble rolls through inside.
• Turns. Where one channel ends, put a card wall for the marble to hit, then start the next channel going back the other way, a little lower.
• Drops. A gap where the marble falls into a tube or a funnel below. Drops speed the marble up.

Test the run after every new piece, not just at the end. When the marble jumps out, gets stuck or stops, you have found a failure point. Common fixes: add a wall on the outside of a turn, make a slope steeper where the marble stops, or make one less steep where it flies off.

Then make it a fair test. Choose one change, such as making the first slope steeper, and time five runs before and five runs after. If your goal is the slowest run, the longest time wins.

Safety: Marbles are a choking danger for babies and young children, and in the United States, toy marbles must carry a choking hazard warning by law. Keep marbles away from younger brothers and sisters, never put a marble in your mouth, and count them back into a closed box when you finish. Pick up any loose marbles at once so nobody slips. A grown-up cuts tubes and card with a craft knife; ask before taping anything to a wall.

For the grown-up helping: Masking tape or painter's tape on a box is kinder to walls than packing tape. If your child gets frustrated, suggest building and testing the run from the bottom up: get the last channel working, then add the one above it.

Common misconceptions

• Mistake: a heavier marble always rolls down faster. On a smooth ramp, a heavier marble and a lighter one of the same size reach the bottom at nearly the same time. What matters most is the height they start from.
• Mistake: a marble can roll up higher than it started. Friction and air use up some energy, so it always comes back up a little lower.
• Mistake: the energy disappears when the marble stops. It has turned into other things, such as a tiny bit of warmth from rubbing and the sound of the marble rolling.
• Mistake: a run is finished when you have built it. A run is finished when it works every time. Test after every piece.

Summing up

• In 1638, Galileo described rolling a bronze ball down a long, smooth groove, timing it by weighing water, and repeating it a full hundred times.
• Height gives a marble stored energy. Rolling down turns it into speed.
• A higher start means a faster marble, which pushes a cup further when they bump.
• Friction and air use up some energy, so a marble cannot climb higher than it started. The first hill must be the highest.
• Build a marble run one piece at a time, and test after every piece.
• Marbles are a choking danger for young children. Keep them boxed and out of reach.

Sources

1. Galilei, G. (1914). Dialogues concerning two new sciences (H. Crew and A. de Salvio, Trans.). Macmillan. (Original work published 1638). Internet Archive
2. NGSS Lead States. (2013). 4-PS3 Energy. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org
3. NASA Glenn Research Center. (2024, July 19). Three forces on a glider. Beginner's guide to aeronautics. nasa.gov
4. Legal Information Institute. (n.d.). 16 CFR 1500.19: Misbranded toys and other children's articles. Cornell Law School. law.cornell.edu
5. U.S. Consumer Product Safety Commission. (n.d.). Small parts and choking hazard labeling FAQs. cpsc.gov

A stack of metal discs

On 20 March 1800, an Italian scientist called Alessandro Volta wrote a letter to Joseph Banks, the president of the Royal Society in London. In it he described a strange stack. It was made of discs of zinc and silver, one on top of the other, with pieces of cloth soaked in salty water between them. The American Physical Society describes it as a rather messy stack of as many as thirty discs.

When wires joined the top and the bottom of the stack, electricity flowed through them. The American Physical Society calls Volta's stack the forerunner of the modern battery, the ancestor of every battery you have ever used. The unit we use to measure a battery's push, the volt, is named after him.

In this lesson you will use Volta's invention, in a much tidier form, to light a bulb. First, though, come the rules. Electricity is safe to explore only when you follow them every time.

The battery safety rules

1. Use only store-bought AA, C or D batteries, or a 9-volt battery. These are the batteries for every circuit in this course.
2. Never use electricity from a wall socket. Never put anything into a socket, and never take apart anything that plugs in. Electricity from the wall is much stronger than a battery. A children's hospital in Chicago warns that an electric shock can cause serious injuries, and that children can be hurt by poking metal objects into sockets.
3. Never use coin or button batteries. These are the small, flat, round batteries found in watches, car keys and some toys. The CPSC, the U.S. government's product safety agency, warns that if a child swallows one, it can burn through the throat in as little as two hours. Poison Control explains why: inside the wet throat, electricity flows around the battery and makes a strong chemical that burns.
4. Never cut open, crush, heat or burn a battery, and never throw one in a fire. The battery maker Energizer warns that a damaged battery can leak or burst.
5. Never join the two ends of a battery with just a bare wire, foil or metal. This is called a short circuit. Energizer warns that it can lead to high heat or leaking. Always have a bulb or LED in the loop.
6. Keep 9-volt batteries away from metal. Fire safety experts at the NFPA explain why. A 9-volt battery's two posts are close together. A paper clip, coin, key or piece of foil touching both posts at once can make enough heat to start a fire. Store 9-volt batteries in their packet, or with tape over the posts.
7. Do not mix old and new batteries, or different kinds, in the same circuit. Energizer advises against it.
8. A grown-up stays with you whenever batteries are out, and puts them away afterwards.

For the grown-up helping: If you ever think a child has swallowed a coin or button battery, go to the nearest emergency department immediately; do not wait for symptoms. In the United States, the National Battery Ingestion Hotline is 800-498-8666, and Poison Help is 800-222-1222. Keep devices with coin batteries out of reach, and check that their battery covers are screwed shut.

Remember: AA, C, D or 9-volt batteries only, with a grown-up. Never wall sockets. Never coin or button batteries. Never a bare wire from one end of a battery to the other.

What a battery does

A U.S. government energy website for kids explains that a battery uses two different metals with a chemical between them. Volta used zinc, silver and salty water. One end of the battery is marked with a plus sign, +, and the other end with a minus sign, -. The battery pushes electricity out of one end, around a path, and back into the other end.

Look at the label on an AA battery. It says 1.5 V. The V stands for volts. The number of volts tells you how hard the battery pushes. A 9-volt battery pushes six times as hard as one AA battery.

A single AA battery is, strictly speaking, called a cell. Two or more cells joined together make a battery. In everyday talk, people call both batteries.

Worked example 1: light a bulb

You need:

• 2 AA cells in a battery holder with two wires, or 1 D cell in a holder
• A small flashlight bulb marked about 2.5 volts, in a bulb holder with two screws
• 2 wires with crocodile clips on each end
• A few sheets of tissue paper

Bulb holders, battery holders and clip wires come in cheap school science kits. Choose a bulb whose volts roughly match the battery: a 2.5 volt bulb suits two AA cells. A small bulb on a much bigger battery, such as a 9-volt, can burn out.

1. Screw the bulb gently into its holder.
2. Clip one wire from the battery holder's red wire, the + side, to one screw on the bulb holder.
3. Clip the second wire from the other screw on the bulb holder back to the battery holder's black wire, the - side.
4. The bulb lights.

Trace the path with your finger: out of the battery, along a wire, through the bulb, along the other wire, and back into the battery. It is a loop. The government energy website says it simply: for electricity to flow, it needs a complete path, or electrical circuit. Think of a circuit as a circle that the electricity runs all the way round.

Change one thing: break the loop

Now unclip just one end of one wire. The bulb goes out at once. Clip it back, and the bulb lights again.

Nothing else changed. The battery is still full and the bulb is still good. But with one gap anywhere in the loop, the electricity has no complete path, so it stops flowing everywhere. A loop with a gap in it is called an open circuit. A complete loop is a closed circuit. Turning off a light at home opens the circuit, and turning it on closes it again.

The upshot: A circuit only works as a complete loop. Break it anywhere, and everything in it stops.

Worked example 2: an LED that only works one way

An LED is a tiny light that uses very little electricity. If you use one, choose an LED sold for 3-volt battery projects that comes with a resistor, a small part that stops too much electricity flowing through the LED.

Look closely at an LED's two wire legs. One is longer than the other. The electronics company SparkFun explains that the longer leg is the positive side, and that electricity flows through an LED in one direction only.

1. Connect the long leg, through its resistor, to the + side of the battery, and the short leg to the - side. The LED lights.
2. Now swap the two wires, so the long leg goes to the - side. The LED stays dark.

Nothing is broken. SparkFun notes that an LED connected backwards on a small battery simply will not light. Turn it the right way round again, and it glows. A bulb does not care which way round it goes. An LED does.

The brightness test

Does a second cell make the bulb brighter? Here is a fair test, using tissue paper to measure.

1. Darken the room. Light the bulb with one AA cell (use a single-cell holder, or a holder with one cell and a grown-up's help).
2. Hold one sheet of tissue paper flat, a finger's width above the bulb. Do not let it touch the bulb.
3. Add sheets one at a time, stacked flat, until you can no longer see the glow through them. Count the sheets.
4. Switch off. Now use two AA cells, with the same bulb, the same wires, the same tissue and the same dark room. Count again.
5. Do each three times and use the middle number.

Here is a sample. Your numbers will be different.

More push from the battery made the bulb glow more brightly, so more sheets were needed to hide it. Small bulbs get warm, so switch off between tests and never wrap a bulb in paper.

Common misconceptions

• Mistake: one wire is enough. Electricity needs a complete loop: out of one end of the battery and back into the other.
• Mistake: the electricity gets used up in the bulb. The electricity flows all the way round the loop and back to the battery. What runs out over time is the energy stored in the battery.
• Mistake: small batteries are harmless. A swallowed coin or button battery can burn through a child's throat in as little as two hours. Size does not make a battery safe.
• Mistake: a bigger battery is always better. A 9-volt battery can burn out a small bulb, and a short circuit on any battery makes heat. Match the bulb to the battery.
• Mistake: a wall socket is just a big battery. Wall sockets are far more powerful and can cause serious injuries. They are never part of these experiments.

What you now know

• Alessandro Volta described the first battery, a stack of zinc and silver discs, in a letter of 20 March 1800. The volt is named after him.
• Use only store-bought AA, C, D or 9-volt batteries, with a grown-up. Never wall sockets, never coin or button batteries, and never a bare wire across a battery.
• Never cut open, crush, heat or burn a battery, and keep 9-volt posts away from metal.
• Electricity needs a complete loop called a circuit. One gap anywhere stops it.
• An LED lights only when its long leg goes to the + side.
• Two cells pushed harder than one, and made the bulb brighter.

Sources

1. American Physical Society. (2006, March). This month in physics history: March 20, 1800: Volta describes the electric battery. APS News. aps.org
2. U.S. Energy Information Administration. (n.d.). Science of electricity. Energy Kids. eia.gov
3. U.S. Consumer Product Safety Commission. (2023, September 11). Making families safer from button cell or coin battery dangers: Reese's Law leads to new federal mandatory safety standard. cpsc.gov
4. National Capital Poison Center. (n.d.). Swallowed a button battery? Battery in the nose or ear? poison.org
5. National Fire Protection Association. (2016). 9-volt battery safety [Safety tip sheet]. nfpa.org
6. Energizer. (n.d.). Battery care. energizer.com
7. Poole, N., and bboyho. (n.d.). Light-emitting diodes (LEDs). SparkFun Learn. sparkfun.com
8. Ann and Robert H. Lurie Children's Hospital of Chicago. (n.d.). Electrical safety [Injury prevention flyer]. luriechildrens.org

The thread inside the bulb

On 17 January 1882, Lewis Latimer of New York was granted a United States patent called Process of Manufacturing Carbons. His patent says the invention "relates more particularly to carbonizing the conductors for incandescent lamps."

Those are long words, so let us unpack them. An incandescent lamp is an old-style light bulb, the kind with a thin thread inside. That glowing thread is called a filament. In early bulbs, the filament was made of carbon. Latimer found a better way to make those carbon filaments. A program at MIT that studies inventors says he protected the filaments in cardboard while they were made, so they lasted better.

Notice the word conductors in his patent. That word is the key to this lesson.

Latimer was born in Chelsea, Massachusetts, in 1848. Before his work on lamps, he drew the drawings for Alexander Graham Bell's telephone patent in 1876. In 1890 he wrote a book about electric lighting. In 1918 he joined the Edison Pioneers, a group of people who had worked with the inventor Thomas Edison.

Conductors and insulators

Some materials let electricity flow through them easily. Others do not. A physics textbook from OpenStax describes it this way.

• A conductor is a material that lets electricity move through it easily. Metals are conductors.
• An insulator (say it: IN-suh-lay-ter) is a material that does not let electricity move through it. Glass is an insulator. The rubber and plastic covering on electric cables are insulators too.

A filament is a conductor. Electricity flows through it. But it is so thin that pushing electricity through it makes it very hot, so hot that it glows. A U.S. government energy article explains it. Electricity passing through a filament makes it glow brightly. The more electricity passes through, the hotter and brighter it gets, and the sooner it burns out. That is the bulb in your circuit from Lesson 17.

In short: Conductors let electricity through. Insulators stop it. Circuits need both.

Build a conductor tester

Use your circuit from Lesson 17, with one change: a gap.

You need: 2 AA cells in a holder, a 2.5 volt bulb in a holder, 3 crocodile clip wires, and ten objects to test. A grown-up stays with you, and the battery safety rules from Lesson 17 all still apply.

1. Clip one wire from the battery's + side to one screw of the bulb holder.
2. Clip a second wire from the other screw of the bulb holder. Leave its far end loose. This is test lead A.
3. Clip a third wire to the battery's - side. Leave its far end loose. This is test lead B.
4. Touch the loose clips of A and B together. The bulb lights, because the loop is complete. Pull them apart, and it goes out.

Now you have a tester. Clip both loose ends onto an object, so the object fills the gap in the loop. If the bulb lights, electricity is flowing through the object: it is a conductor. If the bulb stays dark, the object is an insulator.

The bulb is always in the loop, so this is not a short circuit. Even so, only test small, everyday objects. Never test anything that plugs into the wall, and never use a sharp knife as a test object.

The comparison table

Before you test, write a prediction for every object: conductor or insulator? Then test each one and write what the bulb did. Here is a sample. Your results may differ.

Reading the table

Look down the columns. Every metal object lit the bulb brightly. Plastic, rubber, wood, paper and glass all left it dark.

The surprise is the last row. The gray core of a pencil is called lead. But a science team at McGill University points out that pencils do not contain any lead, and never did. The core is a mix of clay and graphite, a form of carbon. Scientists at the University of Washington explain that graphite conducts, like a metal. A thick pencil line on paper can even act like a wire. It conducts much less well than metal, so the bulb only glows dimly. Remember Latimer's filaments? They were carbon too.

A wrong prediction is not a failure. It is the most useful line in the table, because it is where you learned something.

Key idea: You cannot always tell a conductor by looking. Test it, and let the bulb decide.

Why wires wear coats

With a grown-up, look at the end of one of your clip wires, or at a piece of old wire that a grown-up has cut. Inside is shiny metal, usually copper. Outside is a coat of plastic.

The metal inside is a conductor, so electricity flows along it. The plastic outside is an insulator, so the electricity stays inside the wire and cannot jump to anything it touches. Engineers need both: conductors to carry electricity where it should go, and insulators to stop it going anywhere else. A damaged wire with its metal showing is dangerous. Never touch the wires of anything that plugs into the wall.

Build a switch

A switch is a part that opens and closes a gap in a circuit, so you can turn something on and off without unclipping wires.

You need: a piece of card about 5 centimeters by 8 centimeters, 2 paper fasteners, 1 metal paper clip, and your tester circuit.

1. A grown-up pushes two paper fasteners through the card, about 3 centimeters apart, and opens their legs flat on the back.
2. Hook the paper clip around one fastener, under its head, so that the clip can swing.
3. Clip test lead A to one fastener and test lead B to the other.
4. Swing the paper clip so its free end touches the second fastener. The bulb lights. Swing it away, and the bulb goes out.

The metal paper clip is a conductor, and so are the fasteners. When the clip touches both, the gap is closed, and the loop is complete. When you swing it away, the gap opens. The light switch on your wall does the same job, with metal parts hidden inside a plastic case, which is an insulator.

For the grown-up helping: If the paper clip has a plastic coating, it will not work as a switch, which is a good extra discovery. Scrape nothing; just use a bare metal clip. Keep fasteners and clips away from younger children.

Common misconceptions

• Mistake: anything shiny is a conductor. Shiny plastic is an insulator. Dull gray pencil lead conducts. Test, do not guess.
• Mistake: pencil lead is made of lead. It is graphite, a form of carbon, mixed with clay.
• Mistake: insulators are useless in a circuit. Insulators keep electricity on the right path. Every wire you use is wrapped in one.
• Mistake: a switch makes electricity. A switch only opens or closes a gap. The battery does the pushing.

Putting it together

• Lewis Latimer's 1882 patent improved the making of carbon filaments, the glowing threads in early electric lamps.
• A conductor lets electricity flow through it easily. Metals are conductors.
• An insulator stops electricity. Plastic, rubber, glass, wood and paper are insulators.
• A tester circuit with a gap sorts objects: if the bulb lights, the object conducts.
• Pencil lead is graphite, which conducts, but much less well than metal.
• A switch opens and closes a gap in a circuit. Wires have metal inside and an insulating coat outside.

Sources

1. Latimer, L. H. (1882). Process of manufacturing carbons (U.S. Patent No. 252,386). U.S. Patent Office. Google Patents
2. Lemelson-MIT Program. (n.d.). Lewis Latimer. lemelson.mit.edu
3. Urone, P. P., and Hinrichs, R. (2022). 18.2 Conductors and insulators. In College physics 2e. OpenStax. openstax.org
4. Clean Energy Institute, University of Washington. (n.d.). Draw a circuit: Fun with graphite [Lesson plan]. cei.washington.edu
5. McVean, A. (2019, April 8). Under the microscope: Graphite. Office for Science and Society, McGill University. mcgill.ca
6. U.S. Department of Energy. (2015, December 16). How do holiday lights work? energy.gov

Dirty dishes and a promise

In 1886, Josephine Cochran was granted U.S. Patent No. 355,139 for a machine that washed dishes. Her name is often spelled Cochrane, with an e on the end.

Cochran had a problem. Washing dishes by hand was slow and boring, and it often chipped her china. Then, in 1883, her husband died and left her with debts and little money. According to the National Inventors Hall of Fame, she said: "If nobody else is going to invent a [mechanical] dishwashing machine, I'll do it myself."

So she did. In her machine, wire racks held plates, cups and saucers inside a wheel. The wheel sat in a big copper tub. A motor turned the wheel and pumped hot, soapy water up from the bottom over the dishes. Instead of scrubbing, the force of the water did the cleaning. The racks held each dish still, so they did not knock together and chip.

Cochran started a company to make and sell her machines. Years later, the business became part of KitchenAid, a company that still makes dishwashers. In 2006 she was named to the National Inventors Hall of Fame.

That is how this lesson works: four inventors, four everyday problems, four solutions you can still see around you. Each story is checked against the inventor's patent or against the National Inventors Hall of Fame.

The bag that stands up by itself

Margaret Knight was born in 1838. As a teenager working in a cotton mill in New Hampshire, she came up with a safety device for the weaving machines, to stop workers being hurt.

Later she worked at the Columbia Paper Bag Company in Springfield, Massachusetts. In those days, paper bags were flat, like envelopes. Knight invented a machine that cut, folded and glued paper into bags with flat bottoms, all by itself. A flat-bottomed bag can stand up on its own and hold much more. The Hall of Fame says her machine did the work of 30 people.

Then something unfair happened. A man called Charles Annan worked in the shop that was building her machine. He tried to claim the design as his own. In court, he said that a woman could not understand such a machine. Knight proved him wrong by showing her original drawings of the design. She won, and she received her patent in 1871. By the time she died in 1914, she had patented at least 26 inventions.

So what? Knight's drawings won her case. An engineer's careful records are not just for remembering. They can prove whose idea something was.

Snow on the glass

Mary Anderson lived in Birmingham, Alabama, where she built and managed an apartment building. The National Inventors Hall of Fame says her idea came to her while she was riding a trolley car in New York City on a snowy day in the early 1900s. A trolley car is a kind of streetcar.

Her patent shows the problem she set out to solve. Patent No. 743,801, granted on 10 November 1903, describes a device for "removing snow, rain, and sleet from the glass in front of the motorman", the driver. It was worked from inside the car. The driver turned a handle, and the handle moved a spring-loaded arm with a rubber blade across the outside of the glass. The driver could clear the window without opening it or going outside.

Today almost every car, bus and train has windshield wipers. Anderson was named to the National Inventors Hall of Fame in 2011.

Stop in every direction

Garrett Morgan was born in Paris, Kentucky, in 1877. As a young man he moved to Cleveland, Ohio. There he started businesses that fixed sewing machines and made clothes. He also invented a breathing hood. In 1916, an explosion trapped workers in a tunnel full of gas under Lake Erie. Morgan and other volunteers wore his hoods to go in and rescue survivors.

Traffic signals already existed in the 1920s. Morgan's idea was a better one. His patent, No. 1,475,024, was granted on 20 November 1923. It describes a signal that stops traffic in all directions before any one direction is told to go. That short moment when everyone has to stop made crossings safer. The Hall of Fame says he sold the rights to his signal to General Electric for $40,000. He was named to the Hall of Fame in 2005.

What the four have in common

Look at the table. None of them started with a gadget. Each started with a problem that real people had, which is the Ask step of the design process. Each then designed something that worked, drew it carefully, and got a patent.

A patent is a document from the government that says an invention belongs to its inventor. The U.S. Patent and Trademark Office explains that a patent gives the inventor the right to stop other people from making, using or selling the invention without permission. Today a U.S. patent usually lasts 20 years from the day the inventor applies.

The core of it: Inventions that change everyday life usually start with somebody noticing a problem that everyone else had learned to put up with.

Build a model wiper

Test Mary Anderson's idea for yourself. Which blade material clears water best?

You need: a piece of thick cardboard about 20 centimeters square, a clear plastic sheet protector or a clean, flat plastic lid, a strip of card 2 centimeters by 15 centimeters, 1 paper fastener, tape, a drinking straw, a cup of water, and three blade materials: a strip of kitchen sponge, a strip of paper, and a rubber band stretched along a strip of card.

1. Tape the plastic sheet flat onto the cardboard. This is the windshield.
2. A grown-up makes a hole near one end of the card strip and a hole near the bottom middle of the windshield and cardboard.
3. Join the card strip to the board with the paper fastener, so it can swing like a wiper arm.
4. Tape the first blade material along the underside of the arm, so it presses lightly on the plastic.
5. Use the straw as a dropper, as in Lesson 4. Put 10 drops of water in the area the arm sweeps over.
6. Swing the arm across and back three times.
7. Count the drops still on the plastic. Dry the plastic, change the blade, and test again.

Keep the drops, the number of sweeps and the arm the same. Change only the blade. Here is a sample. Your numbers will be different.

The rubber edge pushed the water away instead of soaking it up or smearing it. That is why Anderson's design used a rubber blade, and why wiper blades are still made of rubber.

Safety: A grown-up makes the holes. The legs of a paper fastener are sharp, so open them flat. Wipe up spilled water, and keep the fastener away from babies and toddlers.

For the grown-up helping: The stories in this lesson have been checked against the patents and the National Inventors Hall of Fame. Popular retellings of some inventors add extra details that cannot be confirmed; if your child finds a different version online, it is a good chance to talk about checking sources.

Common misconceptions

• Mistake: inventors are a special kind of genius. These four were ordinary people with ordinary jobs. Anderson managed an apartment building, Morgan repaired sewing machines, and Knight worked in a cotton mill and a bag factory. What they shared was noticing a problem and refusing to put up with it.
• Mistake: a patent means you made the very first one ever. Traffic signals existed before Morgan's patent. A patent can protect a new improvement, like a signal that stops every direction first.
• Mistake: women did not invent things in the 1800s. Knight's patent came in 1871 and Cochran's in 1886, and Knight had at least 26 patents.
• Mistake: an invention is finished once it works. Each of these inventions was improved by other people for years afterwards. Today's dishwashers and wipers look very different from the first ones.

What to remember

• Josephine Cochran patented a dishwasher in 1886, with racks that held dishes still and hot, soapy water that cleaned them.
• Margaret Knight invented a machine for flat-bottomed paper bags, won a court case with her drawings, and got her patent in 1871.
• Mary Anderson's 1903 patent cleared snow, rain and sleet from a streetcar's front glass, worked by a handle inside.
• Garrett Morgan's 1923 signal stopped traffic in every direction before letting one direction go.
• A patent is a government document that lets an inventor stop others from making or selling their invention without permission.
• In the model wiper test, a rubber edge cleared the water best.

Sources

1. National Inventors Hall of Fame. (n.d.). Josephine Garis Cochran. invent.org
2. National Inventors Hall of Fame. (n.d.). Margaret E. Knight. invent.org
3. Lemelson-MIT Program. (n.d.). Margaret Knight. lemelson.mit.edu
4. National Inventors Hall of Fame. (n.d.). Mary Anderson. invent.org
5. National Inventors Hall of Fame. (n.d.). Garrett Morgan. invent.org
6. Anderson, M. (1903). Window-cleaning device (U.S. Patent No. 743,801). U.S. Patent Office. Google Patents
7. Morgan, G. A. (1923). Traffic signal (U.S. Patent No. 1,475,024). U.S. Patent Office. Google Patents
8. U.S. Patent and Trademark Office. (n.d.). General information concerning patents. uspto.gov

A boy, a code and a cartoon

When Louis Braille was 15, he showed the head of his school in Paris a code of raised dots that blind people could read with their fingers. Braille is still read all over the world today.

It is tempting to tell his story like a cartoon. Many people picture every inventor the same way. A lone genius, usually a man in a white coat, is working by himself. Suddenly a light bulb flashes above his head. He has a perfect idea, builds it the same day, and it works first time.

It makes a good cartoon. But is it true? This lesson tests that picture against three real inventors, checked against the American Foundation for the Blind, the National Inventors Hall of Fame and a United States patent. Watch for the places where the picture goes wrong.

Story 1: twelve dots become six

Louis Braille was born in France in 1809. When he was three, he was playing in his father's workshop, where his father made things from leather. The American Foundation for the Blind tells what happened. Louis picked up a sharp tool called an awl to punch a hole in some leather, as he had seen his father do. The tool slipped and hurt his right eye. An infection spread to his other eye, and by the age of five he was completely blind.

At school in Paris, Louis met an idea that was not his own. A French army captain called Charles Barbier had invented a way for soldiers to send messages at night without making a sound. It was called night writing. It used up to 12 raised dots, felt with the fingers, to stand for sounds. But it was based on the sounds of spoken French, so it could not show spelling or punctuation, and it was hard to use.

Louis and his classmates saw both the promise and the problems. Between the ages of about 13 and 16, he worked on a better system in his free time. When he was 15, he felt his code was good enough to share with the head of his school. All told, he spent about three years refining it.

His system uses a cell of six dots, in two columns of three. Any of the six dots can be raised or left flat. The American Foundation for the Blind says that gives 64 different patterns, enough for every letter, number and punctuation mark, and even some whole words. Braille is still used by blind readers all over the world today.

Where does the cartoon go wrong? Braille did not start from nothing. He started from Barbier's idea and improved it. And it took him years, not a flash.

Count the patterns yourself

Here is a way to check that six dots give 64 patterns. One dot can be up or down: 2 patterns. Each extra dot doubles the number of patterns, because every old pattern can now have the new dot up or down.

One of those 64 patterns has no dots raised at all, which is simply a space. That leaves 63 patterns with dots, plenty for the 26 letters of the alphabet and much more.

Story 2: the spray across the bathroom

Lonnie Johnson trained as an engineer at Tuskegee University. The National Inventors Hall of Fame says that in 1979 he left the U.S. Air Force to work as an engineer at NASA's Jet Propulsion Laboratory on the Galileo mission to Jupiter.

He was also working on a project of his own: a new kind of heat pump, a machine for heating and cooling. The Strong National Museum of Play, which runs the National Toy Hall of Fame, explains that his heat pump used water instead of the usual chemical. When he hooked its nozzle up to his bathroom tap, it shot a powerful stream of water across the room. The Lemelson-MIT program says he first had the idea for a high-powered water gun in 1982.

So far, this sounds like the cartoon: a flash of an idea. But look at what came next. He built a prototype, an early working version, with an air pressure chamber and a water tank. He looked for a toy company to make it, and in 1989 he signed an agreement with a company called Larami. The first Super Soaker went on sale in 1990, about eight years after the idea. The toy was added to the National Toy Hall of Fame in 2015, and Johnson was named to the National Inventors Hall of Fame in 2022.

The flash came to someone who was already doing serious engineering, and it took years of work and a whole company to turn it into a toy.

What matters here: Ideas often come suddenly, but they come to people who are already working on problems. Turning an idea into a real thing takes years.

Story 3: who is at the door?

Marie Van Brittan Brown worked as a nurse in New York City. The Lemelson-MIT program explains that her long hours meant she often came home late at night, alone, and she wanted a safe way to see who was knocking at her door.

She worked on the idea with her husband, Albert Brown, an electronics technician. Their patent, U.S. Patent No. 3,482,037, names them both. It was filed in 1966 and granted on 2 December 1969. The patent describes a camera that could see a visitor standing at the door, a way to talk with the visitor, a lock that could be opened from a distance, and an alarm that could alert a guard or police officer.

Lemelson-MIT describes the setup as four peepholes, a sliding camera, television screens and two-way microphones. Today, camera doorbells and home security systems do many of the same jobs.

Where does the cartoon go wrong this time? Brown did not work alone. The patent has two names on it. And her invention started with a real problem in her own life, not with a light bulb over her head.

What the stories show

The point: Real inventing is the design process you have been using all along: notice a problem, build on what others have done, test, improve, and keep going.

Build a squeeze-bottle squirter

Lonnie Johnson's water gun was all about pressure: water squeezed hard comes out fast. Test one thing about squirters: does the size of the hole change how far the water goes?

You need: 3 identical clean plastic drink bottles with screw caps, water, a thumbtack, a wooden skewer, a pencil, a tape measure, chalk, and a grown-up. Do this outside, on a path or a driveway.

1. A grown-up makes a hole in the middle of each cap. Cap 1 gets a small hole from a thumbtack. Cap 2 gets a medium hole, made by pushing a skewer through. Cap 3 gets a bigger hole, made with the point of a pencil.
2. Fill each bottle with water to the same level, and screw on the caps.
3. Draw a chalk start line. Stand behind it.
4. Hold the bottle level, at waist height, pointing along the path. Squeeze hard with both hands, the same way each time.
5. Measure from the start line to where the furthest drops land.
6. Refill to the same level and do three squirts with each bottle. Use the middle number.

The hardest thing to keep fair is the squeeze. One person should do every squeeze, the same way. That is exactly why you do three tries.

Here is a sample. Your numbers will be different.

In the sample, the small hole squirted furthest, because the squeeze could push the water out of it fast. Through the big hole, the water poured out before much pressure could build up. Try your own and see if you get the same pattern.

Safety: Do this outside. Never squirt anyone in the face or eyes, and only squirt people who have said yes to the game. A grown-up makes the holes with the thumbtack, skewer and pencil. Use plain water only, and keep the thumbtack away from younger children.

For the grown-up helping: Popular retellings of these stories sometimes add details that the museums and patents do not support. The versions here stick to what the American Foundation for the Blind, the National Inventors Hall of Fame, the Lemelson-MIT program, the Strong National Museum of Play and the patent record say.

Common misconceptions

• Mistake: inventors work alone. Brown's patent names her and her husband, and Johnson needed a toy company to make the Super Soaker.
• Mistake: good ideas arrive complete, in a flash. Even when an idea arrives suddenly, it takes years of building, testing and improving.
• Mistake: using someone else's idea is cheating. Braille openly built on Barbier's night writing and made it far better. Improving an idea is how most inventions happen. Copying someone's patented invention to sell it is different, and that is what patents prevent.
• Mistake: you have to be grown up to invent. Braille was 15 when he shared his code.

Where this leaves us

• The cartoon of the lone genius with a sudden perfect idea does not match how real inventing happens.
• Louis Braille, blind from the age of five, improved Barbier's 12-dot night writing into a 6-dot code, sharing it at 15.
• Six dots that can each be up or down give 64 patterns.
• Lonnie Johnson's Super Soaker idea came in 1982 during work on a heat pump; the toy went on sale in 1990.
• Marie Van Brittan Brown and Albert Brown patented a home security system with a door camera in 1969.
• In a fair squirter test, change only the hole size, keep the squeeze the same, and do three tries.

Sources

1. American Foundation for the Blind. (n.d.). An accident in the workshop. The life and legacy of Louis Braille. afb.org
2. American Foundation for the Blind. (n.d.). Braille invents his code. The life and legacy of Louis Braille. afb.org
3. National Inventors Hall of Fame. (n.d.). Lonnie Johnson. invent.org
4. Lemelson-MIT Program. (n.d.). Lonnie Johnson. lemelson.mit.edu
5. The Strong National Museum of Play. (n.d.). Super Soaker. National Toy Hall of Fame. toyhalloffame.org
6. Lemelson-MIT Program. (n.d.). Marie Van Brittan Brown. lemelson.mit.edu
7. Brown, M. V. B., and Brown, A. L. (1969). Home security system utilizing television surveillance (U.S. Patent No. 3,482,037). U.S. Patent Office. Google Patents

A grandfather on the freeway

In 2014, Kenneth Shinozuka of New York City was 15 years old. His grandfather had Alzheimer's disease, an illness that damages memory. Scientific American reported that one night a police officer found his grandfather wandering on a nearby freeway in the middle of the night, in his pajamas.

Kenneth's aunt was his grandfather's main caregiver. Kenneth wanted to keep his grandfather safe and make her job easier. So he set out to solve a real problem for his own family: how could the family know, straight away, when his grandfather got out of bed at night?

His answer was a small pressure sensor that could be attached to a foot or a sock. When a person who should be asleep stepped onto the floor, it sent an alert to a caregiver's smartphone. In August 2014, he won a $50,000 award from Scientific American, given through the Google Science Fair, for an invention that could make a practical difference.

Notice where his invention started. Not with a sensor, and not with a phone. It started with a problem that mattered to people he loved. In this lesson you will find a problem worth solving in your own home. In the next lesson you will solve it.

Hunting for problems

Good problems are hiding everywhere. People get so used to small annoyances that they stop noticing them. Your job is to notice again. Here are three ways.

• Watch. Spend a day noticing when someone sighs, grumbles, drops something, trips or searches for something. Write each one down.
• Ask. Interview the people you live with. Ask: What annoys you at home? What do you lose most often? What takes too long? What is hard for you to reach, carry or remember?
• Listen for "every time". When someone says "every time I..." or "I always have to...", there is often a problem underneath.

The science standards for kindergarten to second grade describe this exact step. Engineers ask questions, make observations and gather information about a situation people want to change, to define a simple problem that a new or improved object could solve.

Why this matters: An invention is only as good as the problem it solves. A clever answer to a problem nobody has is not much use.

A problem list

Here is what one family's problem list might look like after a day of watching and asking.

Some problems are bigger than others, and some are safer to tackle than others. The top kitchen shelf is a real problem, but solving it might mean climbing or carrying heavy things, which is not safe for a home invention. The shoe pile happens every day, affects everyone, and can be tackled with cardboard. That makes it a strong choice.

The safety limits for home inventions

Before you choose, check your problem and your ideas against these limits. Your invention must not:

• plug into the wall, or use any battery other than the AA, C, D or 9-volt batteries from Lesson 17, with a grown-up;
• use heat, flames, sharp blades, chemicals or medicines;
• go in or near the mouth of a baby or toddler, or include small parts they could swallow;
• go around anyone's neck, or include long loose cords;
• need anyone to climb, or be able to fall on someone.

If you are not sure, ask a grown-up before you start. A good engineer checks safety at the Ask step, not after something goes wrong.

Turn the problem into criteria and constraints

You met these two words in Lesson 2. Criteria say what the invention must do. Constraints say what limits you must work inside. Here they are for the shoe pile.

Write your criteria so that you can test them later. "Holds 6 pairs of shoes" can be tested: put the shoes in and count. "Looks nice" is harder to test, though you could ask your family to vote.

Imagine at least three ideas

Now brainstorm, as you did in Lesson 2. Write down every idea, even odd ones. Here are three for the shoe pile.

1. The slanted rack. Two shelves of folded cardboard, sloping down, so shoes sit on them heel first.
2. The cubby wall. Six cereal boxes, tops cut off by a grown-up, taped together side by side and two high, one box for each pair.
3. The parking lot. Tape lines on the floor marking a parking space for each person's shoes, with name labels.

Choose with a choosing table

How do you pick? Score each idea against each criterion: 3 means very well, 2 means fairly well, 1 means not well. Then add up the scores. This is called a choosing table.

The cubby wall scores highest, so it is the one to build first. The parking lot never lifts shoes off the floor, so it fails the most important criterion, even though it scores well on the others. If two ideas tie, pick the one you can test soonest.

Notice that the scores are your best judgment, not facts. The tests in the next lesson will show whether your judgment was right.

Bottom line: Start from a real problem, write criteria you can test, imagine several ideas, and choose with a table instead of a hunch.

For the grown-up helping: Let your child choose the problem, even if you can see a neater one. A problem they care about will carry them through the building and testing in the next lesson. Your role is to check the safety limits and to be interviewed honestly.

Common misconceptions

• Mistake: you need a big, world-changing problem. Kenneth Shinozuka's invention started with one person in his own family. Small, real problems are the best place to start.
• Mistake: start with a cool gadget and find a use for it later. Start with the problem and the people who have it. The gadget comes last.
• Mistake: the first idea is the one to build. Imagine at least three, then choose with criteria.
• Mistake: criteria and constraints are the same thing. Criteria are what it must do. Constraints are the limits you must work inside.

The takeaway

• In 2014, 15-year-old Kenneth Shinozuka won a Scientific American award for a sensor that alerted his family when his grandfather got out of bed at night.
• Find problems by watching, asking and listening for "every time".
• Check every problem and idea against the safety limits for home inventions.
• Write criteria you can test, and constraints you must work inside.
• Imagine at least three ideas, then score them in a choosing table.

Sources

1. Scheer, R., and Sneed, A. (2014, October 1). Teen wins big for his sock invention. Scientific American. scientificamerican.com
2. DiChristina, M. (2014, October 9). Scientific American Science in Action winner Kenneth Shinozuka. Scientific American. scientificamerican.com
3. NGSS Lead States. (2013). K-2-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org
4. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org

A glue that did not stick very well

In 1968, a chemist called Spencer Silver, working at the company 3M, made a new kind of glue. It was a weak glue, sticky enough to hold paper but easy to peel off again. The Lemelson-MIT program, which studies inventors, says it was not an immediate success. Nobody could find a good use for it, and it might have stayed on a shelf for good.

Silver kept showing it to people. For about five years, he talked about it to the people he worked with, informally and in meetings. One of them was Art Fry.

Fry sang in his church choir, and he had a problem. The paper bookmarks in his hymn book kept slipping out, sometimes falling onto the floor. A bookmark made with Silver's weak glue would stay in place, and peel off without tearing the page.

Fry wrote up his idea and passed samples around his workplace. At first, the bosses worried that the product would seem wasteful. But, Lemelson-MIT says, the staff could not get enough of the samples Fry was passing around. It took about five more years to get every detail right and to design machines that could make the notes. In 1980, Post-it Notes went on sale across the United States.

That story is this lesson in miniature: a real problem, a prototype, testing with real users, years of improving, and sharing. Now it is your turn. You will take the idea you chose in Lesson 21 and build it, test it, improve it and share it.

Step 1: build a prototype

A prototype is an early working version of an invention, built so you can test the idea. It does not have to be pretty. It has to be good enough to test.

This lesson follows the cubby wall from Lesson 21, a shoe rack made of cereal boxes, from start to finish. Use it as a pattern for your own invention.

You need: 6 empty cereal boxes of the same size, a large flat piece of corrugated cardboard for a back board, tape, safety scissors, a ruler, a pencil, and your engineering notebook with your plan.

1. A grown-up cuts the top flaps off each cereal box with a craft knife or scissors.
2. Lay three boxes on their sides in a row, open ends facing forward, and tape them together. This is the bottom row.
3. Tape three more boxes on top in the same way. This is the top row.
4. Tape the whole block onto the cardboard back board.
5. Stand it beside the front door, against the wall.

Safety: A grown-up does all cutting with a craft knife, and any hot gluing. Use safety scissors for paper and thin card. Keep tape, scissors and small parts away from babies and toddlers. Check your invention against the safety limits from Lesson 21: nothing that plugs in, no heat, nothing that could fall on someone.

Step 2: test against every criterion

Go back to your criteria and test each one separately. Here are the criteria for the cubby wall, and a test for each.

The test with the 5-year-old is called a user test: you watch the real person the invention is for, using it the way they really would. Do not explain how to use it first. Just watch. Art Fry's samples were a user test too: real people used them and wanted more.

Two criteria failed. That is normal, and it is useful. Each failure tells you exactly what to improve.

Step 3: improve, one change at a time

Improvement 1: the lean. The full top row made the block top-heavy, and the cardboard back board bent. Remember Lesson 5: triangles hold their shape. Tape a triangle of folded cardboard at each side, joining the back board to the floor board, like a brace. Test again for a week. Result: no lean.

Improvement 2: the reach. The top row was too high for the smallest user. Give the bottom row to the youngest people in the house, and stick a picture label on each cubby so a child who cannot read yet can find their own. Run the user test again. Result: the 5-year-old put shoes away three times out of three.

Notice that you made one change, tested it, then made the next. If you had changed both things at once and something went wrong, you would not know which change caused it.

Worth holding on to: Test against every criterion, fix the biggest failure first, and change one thing at a time.

A new situation flips the result

Then it rained. Everyone came home in wet boots and pushed them into the cubbies. By the next morning, the bottoms of the cereal boxes were soft and soggy, and one had sagged.

The design had passed every test. But it had never been tested with wet shoes. You learned in Lesson 4 that card goes soft when it soaks up water. A new situation needs a new test, and a new improvement.

Imagine fixes: line the bottom of each cubby with aluminum foil, or slide a clean plastic food tray into the bottom of each one. Both are waterproof. Choose one, build it, and test it fairly: pour the same small amount of water into a lined cubby and an unlined one, wait an hour, and press the bottoms. The lined cubby stays firm.

Real engineers face this all the time. A design that works in the test room meets something new in the real world, so they test again and improve again. That is why the design process is a loop.

Step 4: share it

Engineers share what they make, so other people can use it, check it and build on it. Plan a five-minute show-and-tell for your family. Cover these points.

1. The problem. What was wrong, and who had the problem?
2. The criteria and constraints. What did your invention have to do, and what limits did you work inside?
3. The ideas. Show your three ideas and your choosing table.
4. The prototype. Show it working.
5. The tests. Show your results table, including what failed.
6. The improvements. What did you change, and what happened when you tested again?
7. What next. What would you improve if you had more time?

Be honest about failures. A presentation that only shows successes hides the most interesting part, and it makes the next engineer repeat your mistakes. Your engineering notebook, with its dates, drawings and tables, is the record that shows your work is real. It was Margaret Knight's drawings that proved her invention was hers.

For the grown-up helping: Ask questions during the show-and-tell rather than giving praise alone: what surprised you, which test was hardest, what would you change next? Then, if the invention is safe and useful, actually use it for a few weeks. Real use is the final user test.

Key idea: An invention is never just built. It is built, tested, improved, tested again and shared.

Common misconceptions

• Mistake: a prototype has to look finished. A prototype only has to work well enough to test the idea. Polish comes later, if at all.
• Mistake: if it works for you, it works. Test it with the real user. The cubby wall worked for a tall person and failed for a 5-year-old.
• Mistake: once it passes the tests, it is done. New situations, like wet boots, can reveal new failures. Keep watching it in real use.
• Mistake: you should hide your failures when you share. Failures and how you fixed them are the most useful part of an engineer's report.

Pulling it together

• Post-it Notes came from a weak glue made in 1968 and a slipping bookmark, and took years of sharing, testing and improving before going on sale in 1980.
• A prototype is an early working version, built to test an idea.
• Test against every criterion, including a user test with the real person the invention is for.
• Fix the biggest failure first, and change one thing at a time.
• When the situation changes, test again: the design process is a loop.
• Share honestly: the problem, the criteria, the ideas, the tests, the failures, the improvements and what comes next.

Sources

1. Lemelson-MIT Program. (n.d.). Art Fry and Spencer Silver. lemelson.mit.edu
2. EiE Team. (2020, November 27). What is the engineering design process? EiE Blog, Museum of Science, Boston. yesblog.mos.org
3. Museum of Science, Boston. (n.d.). Engineering design process. Youth Engineering Solutions. yes.mos.org
4. NGSS Lead States. (2013). 3-5-ETS1 Engineering design. Next Generation Science Standards: For states, by states. National Academies Press. nextgenscience.org