Elementary Science: Life, Earth & Physical

One leaf, ten things to notice

Ask a grown-up to choose a familiar, safe leaf, or study a leaf through a window. Do not pick or touch an unknown plant. Now hold it close to your eyes and count. How many lines run through it? Is the top shinier than the bottom? Is the edge smooth, or does it have little teeth? Is it the same green all over, or lighter near the middle?

Most people would say a leaf is just green and flat. Someone who looks for a whole minute may notice details they missed at first. That is the difference between glancing at something and observing it, and observing is the first job of a scientist.

What is a scientist?

So who gets to be a scientist? A scientist is simply a person who asks questions about the world and looks carefully to find the answers, and that is not just a grown-up in a white coat.

A scientist can be a kid watching ants on the sidewalk. A scientist can be you, wondering why the sky is blue.

Science is not just a big pile of facts. Science is a way of finding things out. Anyone who is curious can do it.

Key idea: A scientist is a curious person who asks questions and looks closely.

Start with a question

So where does real science begin? Almost always, with a question, something you find yourself wondering about.

Good questions often start with these words:

• Why does the sky turn dark at night?
• How does a caterpillar turn into a butterfly?
• What happens if I put a plant in a dark closet?

There are no silly questions in science. Every big discovery started with someone being curious.

What matters here: Science begins when you wonder and ask a question.

Look closely: observing

Once you have a question, what comes next? A scientist observes: they gather information with senses and tools. In these activities, look and listen first. Touch only objects a grown-up has checked; do not taste experiment materials or sniff unknown substances.

You have five amazing tools for this. Five familiar senses are sight, hearing, smell, touch, and taste. Your body also senses things such as balance. Use the senses available to you; tools and another person's description can help.

Say you hold a lemon. It is yellow. It is bumpy. It smells fresh. You may remember its sour taste from a meal. For this observation, you do not need to eat anything.

Good observing is slow. If I hand you a leaf, do not just say it is green. Keep looking. Is it dark green or light green? Are there tiny lines in it? Is one side shiny? A careful observer can find many things about one small leaf.

Worth holding on to: To observe is to use your senses to notice things carefully.

An observation is not a guess

Here is a big idea. An observation is something you really sense. Wet grass is an observation. You can see it and feel it.

A guess is something you think might be true because of what you saw. You might think it rained last night. That is a guess.

Maybe you are right. Or maybe a sprinkler made the grass wet, not rain. A guess can be wrong even when the observation is right.

Good scientists keep observations and guesses in two separate baskets in their minds.

The point: What you saw is an observation. What you think it means is a guess.

The scientific method

So how do scientists put questions, guesses, and observations all together? They use several connected practices, often called the scientific method. The list below is one useful plan, not a rule that every scientist follows in the same order. It sounds fancy, but it is really just a smart way to answer a question. Here are the steps in easy words:

1. Ask a question about something you notice.
2. Suggest a testable explanation, called a hypothesis. Then predict what you expect to observe if it is right.
3. Test your guess by trying something or watching closely.
4. Observe and write down or draw what really happens.
5. Share what you learned with others.

The upshot: This plan helps organize an investigation; real science can revisit or combine its practices.

What scientists mean by a theory

Two science words get mixed up all the time, so let us get them straight now.

A hypothesis is a proposed explanation that can be tested. It draws on what you have noticed. Use it to make a prediction about an observation, then compare that prediction with evidence.

A theory in science is something much bigger. It is an explanation that has been tested again and again, by many different people, in many different places, and is supported by a wide range of evidence. Scientists still test its limits and may revise it when new evidence requires a change. There is a theory of how tiny germs make people sick. There is a theory of how mountains get pushed up. Those are not hunches somebody had one afternoon. They are the best explanations we have, built out of piles and piles of evidence.

So when a person says something is 'only a theory,' they are using the everyday word, which means a hunch. In science the word means almost the opposite. A theory must explain evidence and make testable predictions; its name does not make it beyond question.

Remember: A hypothesis is a testable proposed explanation. A scientific theory is a broad explanation supported by evidence, and it can still be refined.

Let us try it: the sunny window

Let us walk through the steps, one at a time.

• Ask: How does light affect bean seedlings' height and leaf color?
• Guess: I predict that seedlings in light will have greener leaves.
• Test: Put one bean plant on a sunny windowsill. Put another in a dark corner. Give both the same pot, the same soil, and the same water. Only the light is different.
• Observe: Each day, measure height and describe leaf color. A seedling in darkness may stretch taller but look pale; taller does not always mean healthier.
• Share: Tell your family what you found. Show them your chart.

A result that disagrees with your prediction can be useful. But equipment can fail and measurements can be mistaken, so check the setup before deciding what the result means.

Key idea: You can test a real question by trying it and watching what happens.

A safe home experiment

Try this. Get one small object from your kitchen, like an apple or a cracker.

1. Look at it. Write down two things you see.
2. Smell it. Write down what it smells like.
3. Touch it. Is it smooth or bumpy?

You will be surprised how much you can notice about one small thing. That is what scientists do all day.

What matters here: Slow, careful looking helps you learn a lot.

Sort it: observation or guess?

Read each one and say observation or guess. Then check.

• The ice cube is smaller than it was. Observation. You can see the size.
• The ice cube shrank because the room is warm. Guess. You did not see the warmth do it.
• The dog is lying by the door. Observation.
• The dog wants to go outside. Guess. Maybe he just likes the cool floor.
• There are 12 ants on the sidewalk. Observation, because you counted them.
• The ants are marching to a picnic. Guess.

Guesses are not bad at all. Scientists guess constantly. The trick is knowing which is which, so you know what still needs testing.

Worth holding on to: Guesses are useful, as long as you know they are guesses.

Words that help you observe

Careful observing gets easier when you have good words ready to use.

• Size and shape: tiny, wide, curved, pointed, round, flat
• Color: pale, dark, spotted, striped, shiny, dull
• Feel: rough, smooth, sticky, bumpy, soft, cold
• Sound: crunchy, quiet, buzzing, sharp
• Number: how many? Counting is observing too.

Choose an object that a grown-up has checked. Describe it using one word from each row. That is a real scientific observation.

The point: Better describing words make better observations.

Common misconceptions

• Thinking only grown-ups in white coats are scientists. A curious kid is a scientist too.
• Thinking a wrong guess means you did something wrong. A result that disagrees with a guess is worth checking.
• Thinking observing means only looking with your eyes. You can also listen, smell, and touch.
• Mixing up what you saw with what you think it means.

Recap

• A scientist asks questions and observes to learn about the world.
• Science starts with a question that you can test.
• To observe is to gather information carefully using available senses and tools.
• An observation is what you sense. A guess is what you think it means.
• Good describing words make your observations sharper.
• One useful investigation plan is: ask, predict, test, observe, share. Scientists also use other approaches.

Sources

1. NASA Space Place. What Is Science?. Sections on testing hypotheses and scientists not always following one method.
2. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

Two ice cubes and a warm hand

Put two ice cubes on two small plates. Hold one plate between your hands. Leave the other one sitting on the table. Which cube melts first?

You may have a prediction. Here is the harder question: how would you test it to someone who did not believe you? You would have to show that the only thing different between the two cubes was your warm hands. Same size cube, same kind of plate, same room, started at the same moment. That is what an experiment is: a planned comparison that helps you check an explanation. Even a careful test can contain differences you missed.

What is an experiment?

An experiment is a test you plan to find an answer. In this lesson's simple comparisons, you change one condition on purpose and observe the result.

Scientists do experiments to check their guesses. You can do them with things around your house.

Remember: An experiment is a planned test to answer a question.

What makes a test fair?

Our beginner comparisons use a fair test: change one condition and keep other important conditions alike. More advanced experiments can deliberately study several changes together.

Say you want to know if plants grow better with water or with soda. You give one plant water and one plant soda.

To be fair, both plants need the same pot, the same soil, the same light, and the same sunny spot. Only the drink is different.

If you changed two things at once, you would not know which one caused the change. That is why we change just one.

Bottom line: For these simple comparisons, change one condition and keep other important conditions alike.

The one thing you change

A variable is something that can vary. The part you change on purpose is the changed variable; the result you measure can vary too. In these simple comparisons, choose one changed variable and keep other important conditions alike.

In the plant test, the drink is the variable. Water or soda, that is the thing you are testing.

Why this matters: The changed variable is the condition you deliberately change. The result you measure is another variable.

Measure and write it down

Scientists do not just look once. They measure, which means to find out how much or how big using a tool.

You can measure with a ruler, a cup, a clock, or a scale. Measuring turns a guess like taller into a number like 12 centimeters.

Then write it down in a chart or a notebook. Writing helps you remember and compare.

• Day 1: 5 centimeters
• Day 3: 7 centimeters
• Day 5: 9 centimeters

In short: Measuring and writing it down helps you see what really happened.

Do it more than once

You have measured and written everything down. But is one try ever enough? One try can happen by luck. If you flip a coin once and get heads, that does not prove coins always land on heads.

So scientists repeat their tests. To repeat means to do it again to check. If you get the same answer many times, you can trust it more.

The core of it: Doing a test more than once helps you trust the answer.

A safe home experiment: sink or float

Here is a fun one you can do with a bowl of water. Ask a grown-up if you can use the sink.

1. Fill a bowl with water.
2. With a grown-up, gather clean objects too large to swallow, such as a plastic spoon and a wooden block. Keep water away from electricity and empty the bowl afterward.
3. Before you drop each one, guess: will it sink or float?
4. Drop it in and watch. Write down what really happened.

You will find that some heavy-feeling things float and some light things sink. That is a great new question to explore.

Remember: Guessing first, then testing, is exactly how scientists work.

Stay safe

Real scientists stay safe. So do you.

• Ask a grown-up before you start.
• Do not taste experiment materials, even if they began as food.
• Clean up spills so no one slips.
• Wash your hands when you finish.

Bottom line: Being safe is part of being a good scientist.

Fair test or not?

Read each plan. Decide whether it is a fair test. Then check.

• Two plants. One gets water, one gets soda. Same pot, same soil, same window. Fair. Only the drink is different.
• Two plants. One gets water in a sunny window, one gets soda in a dark closet. Not fair. Two things changed, so you cannot tell which one mattered.
• Two paper airplanes, same paper, same folds. You throw one gently and one hard. Fair, if you are testing throwing strength.
• Two paper airplanes made from different paper, thrown by two different people. Not fair. The paper and the thrower both changed.

For these beginner tests, ask whether another difference could explain the result. If so, revise the plan to make the comparison clearer.

Why this matters: In these simple comparisons, more than one changed condition can make it hard to identify the cause of a difference.

A chart you can copy

Charts keep your results tidy. Here is a simple one for the sink or float test.

Look at the grape row. The guess was wrong, and that is the most interesting row on the whole chart. It gives you a brand new question: why does a grape sink when an apple floats?

Always write your guess before you test. Otherwise it is far too easy to say afterward that you knew it all along.

In short: Write your guess before you test, and keep the rows where you were wrong.

Another safe experiment: the melting race

Use room-temperature water and refrigerator-cold water, with a grown-up. Do not use a stove, kettle, microwave, or hot tap.

1. Put one ice cube in a cup of cool water and one in a cup of room-temperature water. Use equal water amounts, and let a grown-up check both cups.
2. Guess which one will melt first, and write your guess down.
3. Watch both cups. Use a clock or count slowly.
4. Write down how long each one took.

What is the variable here? The water temperature. Everything else, including the size of the ice cubes and the size of the cups, should be the same.

Try it a second time with two fresh cubes. Two tries give you more to compare than one, but agreement does not rule out a shared mistake.

The core of it: Keep other important conditions alike when comparing the effect of one changed condition.

What if my guess was wrong?

Then you learned something! A wrong guess is not a failed experiment.

Real scientists guess wrong all the time. When the result surprises them, they get excited, because a surprise means the world works differently than they thought.

So when your result does not match your guess, do not erase it. Write down what really happened, and then ask a new question about why.

Remember: A surprising result is worth checking; it may reveal something new or a problem with the test.

Common misconceptions

• Changing two things at once. Then you cannot tell what caused the result.
• Only looking once. Do it again to be sure.
• Forgetting to write things down. Your memory can trick you.
• Writing your guess after you see the result.
• Thinking a surprising result is a mistake. A surprise needs investigation.

What to carry forward

• An experiment is a planned test to answer a question.
• Our simple fair tests change one condition and keep other important conditions alike.
• The condition you deliberately change is the changed variable.
• Measure with tools, and write your guess down before you test.
• Repeat your test so you can trust the answer.
• A wrong prediction is not a reason to erase a result.

Sources

1. NASA Space Place. What Is Science?. Sections on testing hypotheses and scientists not always following one method.
2. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

A dog, a rock, and a candle flame

A dog is alive. A rock is not. That one is easy. Now try a harder one: a candle flame.

A flame moves. It grows bigger when you give it more wax. It gets hot, it uses up the air around it, and if its oxygen supply runs out, it goes out. This is a reading example, not an activity: do not light or cover a flame. That is four things a flame has in common with living creatures. So is a flame alive?

It is not, and working out exactly why not is how scientists ended up with a careful list of what counts as living.

What is a living thing?

A living thing is an organism, such as a dog, tree, or bacterium. Organisms are made of cells, use energy and materials, and respond to their surroundings. Their life cycles connect one generation to the next; an individual need not have babies to be alive.

Dogs, cats, trees, flowers, fish, birds, and people are all living things. You are a living thing.

Many tiny things, such as bacteria, are living cells. Not every germ is a living cell: viruses are different.

The upshot: Living things are made of cells and use energy and materials to stay alive.

What is a nonliving thing?

A nonliving thing is something that is not alive. It does not grow on its own, and it does not need food or water.

Rocks, water, toys, chairs, spoons, and clouds are nonliving. A teddy bear is nonliving, even though it looks like an animal.

Key idea: A nonliving thing is not alive and does not need food or water.

Clues to life

Living things share a pattern of features. No single action, such as moving, settles the question.

• Cells: A cell is a tiny living unit. A bacterium has one cell; a dog has many.
• Energy and materials: Animals eat food. Green plants use light to make sugars. Cells need water and materials to work.
• Growth and response: A puppy grows into a dog. A plant can grow toward light. An adult does not have to keep getting bigger to be alive.
• New generations: Living populations continue through reproduction. A young animal, or one that cannot have offspring, is still alive.

For pets, think about suitable food, water, oxygen, and shelter. Fish take oxygen from water. Some microorganisms live without oxygen, so breathing air is not a test for all life.

What matters here: Look at cells and life processes together, not one visible clue.

Tricky ones

Some things can trick you. A car moves and needs gas, but it is nonliving. It cannot grow or have baby cars.

A river moves too, but it is nonliving. Moving alone does not mean something is alive.

A seed looks still, like a tiny stone. A healthy seed contains a living young plant. It may begin growing when it gets the right water, oxygen, and temperature. Some seeds are dead or need other conditions, so not every seed will sprout.

A dry, dead leaf was once part of a living tree. A freshly picked leaf can still contain living cells, so falling or picking does not instantly make every cell dead.

The point: Moving alone does not make something alive. Look for growing, eating, and making more.

A safe home experiment: sort your room

Try this at home. It is easy and fun.

1. Walk around one room with a grown-up.
2. Point to five things. For each one, say living or nonliving.
3. For living things, name one thing it needs, like food or water.
4. A pet and a houseplant are living. A lamp and a pillow are nonliving.

The upshot: You can sort the whole world into living and nonliving things.

Living, nonliving, or once living?

What about a wooden chair? It is not alive, but it did not start out like a rock either. There is a third group worth knowing: some things are not alive now, but they used to be part of something living. We call those once living.

• A wooden chair. Once living, because it was made from a tree.
• A cotton shirt. Once living, because cotton grows on a plant.
• A plastic cup. Nonliving. It was never alive.
• A freshly picked apple. It still contains living cells, even though it is no longer attached to its tree.
• A ladybug on a leaf. Living.
• A metal spoon. Nonliving.

Ask whether it is alive now and whether it came from an organism. 'Once living' describes nonliving material that came from life; it is not separate from the broader nonliving group.

Key idea: Wood, cotton, and paper are once living, because they came from plants.

Check the seed for yourself

A seed sits still like a pebble, so it is easy to think it is not alive. Here is a safe way to prove it is.

1. Ask a grown-up for a few dried beans, like pinto or lima beans.
2. Fold a paper towel, wet it so it is damp but not dripping, and put it in a clear jar or a zip bag.
3. Tuck 3 beans between the towel and the side of the jar so you can see them.
4. Put it somewhere warm and check every day. Keep the towel damp.

A viable bean may sprout in several days, but timing varies. Do not eat the beans or sprouts. A grown-up should discard moldy material without opening it, and everyone should wash their hands.

A pebble would never do that, no matter how long you waited. The bean did it because it was alive the whole time, just resting.

What matters here: A sprouting seed shows that its young plant was alive. A seed that does not sprout needs further investigation.

Sort a whole room

Try this with a grown-up. Walk around and point at ten things. For each one, say living, nonliving, or once living.

Here is a sample answer key from a kitchen.

• A houseplant: living.
• A freshly picked banana: plant tissue with living cells.
• The refrigerator: nonliving.
• A wooden spoon: once living.
• A pet fish: living.
• Water in a glass: nonliving.
• A paper napkin: once living, because paper comes from trees.

Some things will start arguments, and that is fine. Explaining why you chose a group is the real science here.

Worth holding on to: Saying why you sorted something a certain way matters more than the answer itself.

Common misconceptions

• Thinking that anything that moves is alive. Cars and rivers move but are not alive.
• Thinking a seed is nonliving because it sits still. A healthy, viable seed contains a living young plant.
• Thinking a teddy bear is alive because it looks like an animal.
• Thinking 'nonliving' means 'never came from life.' Wood and paper are nonliving materials that came from trees.
• Forgetting that plants are living things too.

The short version

• Living things are made of cells and carry out life processes; not every individual reproduces.
• A nonliving thing is not alive and does not need food or water.
• Once living things, like wood and cotton, came from plants or animals.
• Living cells need water, energy, and suitable conditions; the needs of different organisms vary.
• Moving alone does not make something alive.

Sources

1. OpenStax. Biology 2e, 1.2 Themes and Concepts of Biology. Properties of Life and Levels of Organization.
2. NC State Extension. Postharvest Engineering: Introduction. Opening discussion of living harvested tissues and continued respiration.
3. University of Minnesota. The Science of Plants, 9.2 Seed Physiology. Seed in a quiescent state; Germination; External conditions.

What is folded up inside a bean

With a grown-up, soak an untreated dried bean overnight and have the adult gently open it. Do not eat experiment beans; wash your hands afterward. It comes apart into two thick halves, and tucked between them is something tiny, pale and curled: a little hook with a point at one end.

That hook is the plant. The two thick halves are its packed lunch, the food it will live on until it can make its own. The whole thing was in there the entire time, dry and still, waiting for water.

The parts of a plant

Most plants have the same main parts. Each part has a special job.

• Roots: The roots grow down into the soil. They drink up water and hold the plant in place.
• Stem: The stem holds the plant up. It carries water from the roots to the leaves.
• Leaves: The leaves catch sunlight and make food for the plant.
• Flowers: Many plants grow flowers that make seeds for new plants.

The core of it: Roots drink water, the stem holds the plant up, and leaves catch sunlight.

How plants make food

Here is something amazing. Plants make their own food. They do it in a process called photosynthesis.

Photosynthesis is when a plant uses sunlight, water, and air to make its own food. The green leaves do most of the work.

Think of a leaf like a tiny kitchen. It takes in sunlight from above, water from the roots, and air from around it. Then it makes sugar, which is the plant food.

Plants even give off fresh air for us to breathe while they do this. That is why plants and trees are so important.

Remember: Photosynthesis is how plants use sunlight, water, and air to make their own food.

What plants need to grow

To grow big and healthy, a plant needs four things:

1. Sunlight, to make food.
2. Water, which the roots drink up.
3. Air, which the leaves use.
4. Mineral nutrients, which plants often take from soil. Roots also need oxygen and support. Plants can grow without soil when another setup supplies these needs.

If a plant is missing one of these, it may grow weak or turn yellow. Follow the growing instructions for your plant; too much water can leave roots without enough oxygen.

Bottom line: Most green plants need light, water, air, and mineral nutrients to grow.

A flowering plant's life cycle

A plant grows in steps. These steps are called a life cycle. A life cycle is the path a living thing takes from its start to when it makes new life.

1. A seed rests in the soil. A seed is a tiny package that holds a baby plant.
2. With water and warmth, the seed sprouts. A little root goes down and a green shoot goes up.
3. The young plant grows a stem and leaves.
4. A mature flowering plant can make flowers. Ferns and mosses have different life cycles and do not make flowers.
5. The flowers make new seeds, and the cycle starts again.

Why this matters: A plant grows from a seed, into a plant, into flowers, and then makes new seeds.

How seeds travel

Seeds need to move away from the parent plant to find their own space, sun, and soil. Nature helps them travel.

• Wind blows light seeds, like a dandelion puff, through the air.
• Animals eat fruit and drop the seeds far away.
• Some seeds have little hooks that stick to fur or socks.
• Water carries some seeds down a stream.

In short: Wind, animals, and water help seeds travel to new places.

A safe home experiment: grow a bean

You can watch a seed grow. Ask a grown-up for a dried bean and a clear cup.

1. Fold a wet paper towel and put it inside the clear cup.
2. Tuck a bean between the towel and the side of the cup, so you can see it.
3. Keep the towel damp and put the cup in a warm, bright spot.
4. Watch each day. A viable bean may produce a root and shoot within about a week. Keep it out of hot direct sun. Do not eat it; let an adult discard moldy material and wash your hands.

The core of it: With suitable water, oxygen, and temperature, a viable bean can sprout. Germination time varies.

You eat plant parts every day

Open the refrigerator and you will find every plant part in there. Try naming which part each food is.

• Carrot: a root. That is why it grows underground.
• Celery: a leaf stalk, called a petiole. It connects a leaf blade to the stem and contains tubes that carry water.
• Lettuce or spinach: leaves. Flat and wide, perfect for catching sunlight.
• Broccoli: flower buds, picked before they open.
• Sunflower seeds and peas: seeds.
• Potato: a special underground stem that stores food for the plant.

The potato surprises most people. It grows underground like a root, but it is really a swollen stem where the plant keeps its stored food.

Remember: Vegetables include many plant parts, including roots, stems, leaves, flowers, seeds, and fruits such as tomatoes.

See water move through celery

This one shows you the plant's water pipes. Ask a grown-up to help, because it needs a knife.

1. A grown-up cuts the bottom off a stalk of celery with leaves still on.
2. Fill a clear cup with water and stir in about ten drops of food coloring. Red or blue shows up best.
3. Stand the celery in the colored water and leave it overnight.
4. The next day, look at the leaves and the cut end.

You may see thin colored lines running up the leaf stalk, and the leaf edges will have a tint of color. The water traveled from the bottom all the way to the top.

Ask a grown-up to slice across the leaf stalk. The little colored dots you see are the tubes that carried the water.

Bottom line: Colored water traces tubes in celery's leaf stalk. This cut stalk has no roots; the test does not directly show root uptake.

Test what a plant really needs

Consider this imagined comparison of three similar plants. Plan and predict on paper; you do not need to deprive a real plant of water or light.

• Plant A: sunny window, watered normally. This is your normal plant to compare against.
• Plant B: dark cupboard, watered normally.
• Plant C: sunny window, no water.

For this imagined comparison, describe what you would check every few days. Plant B may become pale and weak as stored food is used. Plant C may wilt as water is lost. Timing depends on the plant and conditions; these are predictions, not guaranteed results.

Remember to give them the same pot, the same soil, and the same starting size. That is what makes it a fair test.

Why this matters: Comparing similar plants helps test what a condition does. Record differences rather than assuming every plant responds on the same schedule.

Common misconceptions

• Thinking plants eat food from the soil like we eat dinner. Plants make their own food from sunlight.
• Thinking roots are not important because you cannot see them. Roots drink the water.
• Thinking a plant can live in a dark closet. Plants need light to make food.
• Thinking seeds are not alive. A seed holds a baby plant and is living.
• Watering a plant so much that the soil never dries. Roots need some air too.

Where this leaves us

• Plants have roots, a stem, leaves, and often flowers.
• Photosynthesis is how plants make food from sunlight, water, and air.
• Most green plants need light, water, air, and mineral nutrients.
• The vegetables you eat are plant parts: roots, stems, leaves, flowers, and seeds.
• A plant grows from a seed and later makes new seeds.
• Wind, animals, and water help seeds travel.

Sources

1. University of Minnesota. The Science of Plants, 1.3 Plant Parts We Eat. Petiole examples: celery; stems and inflorescences.
2. University of Minnesota. The Science of Plants, 9.2 Seed Physiology. Seed in a quiescent state; Germination; External conditions.
3. NASA Science. Spotlite: Do Plants Need Soil?. Page description, paragraphs 1-2.
4. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

Fur, fat, and feet that do not sink

A polar bear has black skin underneath its white fur, a thick layer of fat, and paws so broad they spread its weight over the snow like snowshoes. Everything about it is built for cold.

A camel has a different combination of helpful features. It has long legs that hold its body up away from burning sand, and it can go for days without a drink. Moving an animal far outside the conditions it can tolerate can put it in danger; survival depends on the species and conditions.

What is a habitat?

A habitat is the place where an animal lives. It is the animal's home in nature.

A habitat gives an animal what it needs: food, water, air, and a safe place to rest and raise babies.

There are many kinds of habitats: forests, deserts, oceans, ponds, grasslands, and icy poles.

The point: A habitat is the place where an animal finds food, water, and shelter.

Different homes for different animals

Each habitat is home to animals that fit it well.

• Ocean: Fish, whales, and crabs live in the salty water.
• Desert: Many camels, lizards, and snakes live in dry habitats. Some deserts are cold.
• Forest: Deer, owls, and bears live among the trees.
• Polar habitats: Wild polar bears live in the Arctic. Some penguins live in Antarctica; others live in warmer regions. They do not naturally share a polar home.
• Pond: Frogs, ducks, and turtles live in and near the water.

The upshot: Different habitats are home to different animals.

How animals fit their homes

Animals have special body parts that help them live in their habitat. These helpful features are called adaptations. An adaptation is an inherited feature that helps an organism survive or reproduce in its environment. It can involve body structure or behavior and develops in populations over many generations.

• A polar bear has thick fur and fat to stay warm on the ice.
• A camel can go a long time without water in the hot desert.
• A duck has webbed feet that work like paddles for swimming.
• A fish has gills to breathe under water.
• A giraffe has a long neck to reach leaves high in the trees.

Key idea: An adaptation is an inherited feature suited to an organism's environment.

What do animals eat?

Animals eat different foods. We can sort them into three groups.

• Herbivores eat plants. A herbivore is a plant eater, like a rabbit or a cow.
• Carnivores eat other animals. A carnivore is a meat eater, like a lion or a shark.
• Omnivores eat both plants and animals. An omnivore eats both, like a bear or a person.

What matters here: Animals can be plant eaters, meat eaters, or both.

How animals stay safe

Animals have clever ways to stay safe from other animals that want to eat them.

• Camouflage is a color or pattern that helps an animal hide. A green frog blends into green leaves.
• A turtle hides inside its hard shell.
• A skunk makes a bad smell to scare others away.
• A rabbit runs fast and zigzags.

Worth holding on to: Animals hide, run, or protect themselves to stay safe.

When homes change

Animals depend on their habitat. If the habitat changes too much, animals can lose their home.

When people cut down a forest or pollute a river, the animals there may have nowhere to live.

This is why keeping habitats clean and safe helps animals. We will learn more about caring for the Earth later.

The point: Animals need their habitats, so keeping habitats safe helps animals.

A safe home activity: watch a habitat

You do not need to travel far to study a habitat. Try this.

1. Sit quietly in a yard, a park, or by a window for five minutes.
2. Watch for animals: birds, bugs, squirrels, or worms.
3. Write down what each one does. Where does it hide? What does it eat?
4. Think about how its body helps it live there.

The upshot: Even a backyard is a habitat full of animals to observe.

Food chains start with the Sun

Every animal needs energy, and almost all of that energy starts in the same place: the Sun.

A food chain shows how energy passes from one living thing to the next. Here is a simple one.

1. The Sun shines on a meadow.
2. Grass uses that sunlight to make its own food.
3. A grasshopper eats the grass.
4. A frog eats the grasshopper.
5. A hawk eats the frog.

Follow the energy. The grass stored some sunlight energy in food. Eating transfers some of that stored energy along the chain; sunlight itself is not the material making up the grass. The hawk is running on sunshine that took a few steps to get there.

Here is an ocean chain: Sun, then tiny floating plants, then small fish, then a bigger fish, then a seal.

Most food webs get their energy from sunlight captured by plants, algae, or some bacteria. But near some deep-ocean vents, microbes use chemical energy to make food without sunlight. Our meadow chain is one example, not a rule for every ecosystem.

Key idea: This meadow food chain starts with grass capturing sunlight, then transfers energy to animals.

Match the animal to the home

For each animal, name a habitat and one adaptation that helps it live there. Then check.

• Penguin. Habitat: cold icy coast. Adaptation: a thick fat layer and stiff flippers for swimming.
• Cactus wren. Habitat: hot desert. Adaptation: it gets most of its water from the food it eats.
• Beaver. Habitat: river or pond. Adaptation: a flat tail for steering and strong teeth for cutting wood.
• Owl. Habitat: forest. Adaptation: huge eyes for seeing at night and soft feathers for silent flying.
• Earthworm. Habitat: soil. Adaptation: a smooth, damp body that slides easily through the ground.

Now try the game backwards. Pick a habitat, like a coral reef, and invent an animal with three adaptations that would help it live there.

What matters here: If you know the habitat, you can often guess what adaptations an animal will have.

Try a camouflage test

This models one way color can affect how easily an observer finds an object. You need small paper squares and a grassy or leafy spot.

1. Cut about 20 small squares of paper: 10 green and 10 bright red or orange.
2. Scatter them all over a patch of grass while a friend looks away.
3. Give your friend 15 seconds to pick up as many as they can.
4. Count how many of each color they found.

Compare the counts. The result depends on the background, lighting, placement, and the observer's vision; the prediction is that harder-to-see colors are found less often.

That is camouflage in action. In nature, the animals that blend in are the ones a hungry predator misses, so more of them survive. Pick up all your paper afterward so the real habitat stays clean.

Worth holding on to: Blending in makes an animal harder to find, and that helps it survive.

Common misconceptions

• Thinking every habitat suits every animal. Animals need conditions they can tolerate.
• Thinking adaptations happen on purpose in one day. They build up slowly over many, many animal lifetimes.
• Thinking camouflage means an animal is invisible. It just helps it blend in and hide.
• Forgetting producers. Plants, algae, and some microbes make food that supports consumers.
• Forgetting that people can change habitats too.

Putting it together

• A habitat is the place where an animal lives and finds what it needs.
• Different animals live in different habitats.
• Adaptations are body parts or habits that help an animal fit its home.
• Animals can be herbivores, carnivores, or omnivores.
• A food chain passes energy from the Sun to a plant and then up to animals.
• Animals hide, run, or protect themselves to stay safe.

Sources

1. NOAA Ocean Exploration. Chemosynthesis Fact Sheet. Pages 1-2, photosynthesis and chemosynthesis.
2. Smithsonian Ocean. The Arctic and The Antarctic. Polar life sections describing polar bear and penguin habitats/adaptations.
3. OpenStax. Biology 2e, 1.2 Themes and Concepts of Biology. Properties of Life and Levels of Organization.

Your heart keeps working while you rest

Sit quietly and rest a hand on your chest. You may feel a gentle beat. If you cannot feel it, you can still follow this reading example; do not press on your neck.

The heart pumps while you are awake and asleep. Its rate changes with activity, age, and other conditions. We will compare observations, not use a home count to decide whether someone is healthy.

Your body is a team

Your body is made of many parts. Some you can see, like your arms and legs. Some are inside, like your heart and stomach.

Groups of parts that work together to do a job are called systems. A system is a team of body parts with one big job.

In short: Your body is a team of parts that work together in systems.

Your bones and muscles

Inside you is a frame of hard parts called bones. Bones are the hard parts that hold up your body and keep its shape. All your bones together make your skeleton.

Bones also keep soft parts safe. Your skull is a bony helmet for your brain. Your ribs make a cage that protects your heart and lungs.

Your muscles are the soft parts that pull on your bones to make you move. Muscles work when you run, smile, or wave.

The core of it: Bones hold you up and keep you safe. Muscles pull on bones to make you move.

Your heart and blood

Your heart is a strong muscle in your chest. Its job is to pump blood all around your body.

Blood carries nutrients from digested food and oxygen to your body's tissues. It also carries carbon dioxide away. Your heart beats all day and all night, without you telling it to.

Put your hand on your chest and you may feel it. Run around, and your heart beats faster to give your body more.

Remember: Your heart pumps blood that carries oxygen and nutrients.

Your lungs and breathing

Your lungs are two soft parts in your chest that fill with air when you breathe. You have two of them.

When you breathe in, your lungs take in fresh air. Your body uses a part of that air, called oxygen, to stay alive.

When you breathe out, some carbon dioxide leaves your body in the air. Exhaled air still contains oxygen; it is not made only of waste gas.

Bottom line: Your lungs bring in fresh air so your body can use it.

Your brain

Your brain is the control center of your body. It sits inside your head, kept safe by your skull.

Your brain lets you think, learn, remember, and feel. It helps control movement and breathing and adjusts heart rate. The heart also has its own electrical cells that start each beat.

Your brain talks to the rest of your body through tiny message lines called nerves. The messages zoom faster than you can blink.

Why this matters: Your brain is the control center that helps you think and move.

Your stomach and food

When you eat, your food goes down to your stomach. The stomach is the part that breaks down food after you swallow.

Digestion starts in the mouth and continues in the stomach and small intestine. Most nutrients enter the blood through the small intestine. The stomach is one part of the team, not the whole process.

In short: Your stomach breaks down food so your body can use it for energy.

Taking care of your body

Your body works hard for you. Here is how to help it stay healthy:

• Eat healthy foods like fruits and vegetables.
• Drink water.
• Run and play to keep your heart and muscles strong.
• Sleep well so your body can rest and grow.
• Wash your hands to keep germs away.

The core of it: Healthy food, water, play, sleep, and clean hands keep your body strong.

A safe home activity: feel your heartbeat

Try this to feel your body at work.

1. Sit still and put your hand on your chest. Feel your heartbeat.
2. If a grown-up says movement is suitable for you, move comfortably in place for a short time. You can also read the example without exercising. Stop if you feel unwell.
3. Put your hand back on your chest. Is your heart beating faster?
4. It beats faster to give your busy muscles more blood.

Remember: Your heart speeds up when you move, to help your body.

Your five senses

Your body gathers information in several ways. Five familiar senses are listed below. Balance and body position are other senses. People use different combinations of senses and helpful tools.

• Sight: your eyes catch light and send a picture to your brain.
• Hearing: your ears catch tiny shakes in the air and turn them into sound.
• Smell: your nose picks up bits floating in the air.
• Taste: your tongue tells sweet, salty, sour, bitter, and savory (umami) apart.
• Touch: your skin feels warm, cold, smooth, rough, and pressure.

Every sense sends its message to the same place: your brain. Your brain puts all five together into one picture of what is happening around you.

Smell and taste are the closest partners. That is why food tastes flat and boring when your nose is stuffy.

Bottom line: Your five senses collect information, and your brain puts it together.

Compare sensory information

With a grown-up, examine a familiar clean wooden spoon. Describe its shape using sight or touch, then gently tap it on a table and describe the sound. Another person can describe details you cannot sense yourself. Do not put it in your mouth.

One sense does not tell you everything. Smell contributes to food's flavor, while taste detects qualities such as sweet or salty. A blocked nose can make familiar food seem different, without removing every taste.

Why this matters: Different senses supply different information, and the brain combines it.

Count your breaths

Here is a quiet activity that shows your lungs at work.

1. Sit still and rest a hand on your belly.
2. Count how many breaths you take in one minute. Ask a grown-up to watch the clock.
3. If a grown-up says it is suitable, move gently in place for a short time. Skip this step if it is not comfortable, and never hold your breath.
4. Sit back down and count your breaths for another minute.

Your second number may be higher. Your muscles were working hard, so they needed more oxygen, and your lungs sped up to deliver it.

Now sit quietly and wait a minute or two. Your breathing slows down all by itself. Nobody told it to. Your brain handled the whole thing without you thinking about it.

In short: Your breathing speeds up when your muscles need more air, and slows down on its own afterward.

Common misconceptions

• Thinking your heart looks like a red paper heart. A real heart is a strong muscle the size of your fist.
• Thinking you breathe only through your mouth. You breathe through your nose too.
• Thinking bones are dry sticks. Living bones are strong and even a little bendy.
• Confusing taste with flavor. Smell contributes to flavor alongside taste, texture, and temperature.
• Thinking your brain rests when you sleep. It keeps your heart and lungs going all night.

What to remember

• Your body is a team of parts that work in systems.
• Bones hold you up and protect you. Muscles make you move.
• Your heart pumps blood. Your lungs bring in air.
• Your brain is the control center.
• Your five senses collect information and send it to your brain.
• Your stomach breaks down food for energy.

Sources

1. NHLBI. How the Heart Works: How the Heart Beats. Electrical system and physical activity sections.
2. NIDCD. Taste Disorders. How does your sense of taste work?.
3. NIDDK. Your Digestive System and How It Works. What happens to the digested food? and small intestine sections.
4. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

One ice cube, two visible states

With a grown-up, put an ice cube on a plastic plate at room temperature. It begins as a solid with its own shape. As it melts, liquid water spreads across the plate.

Leave the plate somewhere it will not spill. Over time, water can enter the air as invisible water vapor. This is evaporation; a stove is not needed. Wipe spills, and do not taste experiment water.

What is matter?

Matter takes up space and has mass, a measure of how much matter is present. Weight is gravity's pull on that mass. Your chair is matter. Water is matter. Even the air around you is matter.

Almost everything you can think of is made of matter. Matter comes in three main forms called states: solid, liquid, and gas.

Worth holding on to: Matter is anything that takes up space and has mass.

Solids

A solid is matter that keeps its own shape. A block, a spoon, and a rock are solids.

If you put a solid in a box, it keeps its shape. A toy car is a car in your hand or in a bowl. It does not spread out.

Solids can be hard like a rock or soft like a pillow, but they all hold their shape.

The point: A solid keeps its own shape.

Liquids

A liquid is matter that flows and takes the shape of its container. Water, milk, and juice are liquids.

Pour a liquid into a round cup, and it becomes round. Pour it into a tall glass, and it becomes tall. A liquid changes shape but keeps the same amount.

Liquids can be poured and spilled. That is why we keep them in cups and bottles.

The upshot: A liquid flows and takes the shape of its container.

Gases

A gas is matter that spreads out to fill all the space it can. Air is a gas. The bubbles in soda are gas.

You often cannot see a gas, but it is there. When you blow up a balloon, gas fills it and makes it puff out.

A gas has no fixed shape or volume. It spreads through its available space; not every gas rises upward.

Key idea: A gas spreads out to fill all the space it can.

Water can be all three

Here is something cool. Water can be a solid, a liquid, or a gas.

• Solid: When water gets very cold, it freezes into ice.
• Liquid: When ice warms up, it melts into water you can pour.
• Gas: Boiling or evaporation changes liquid water into invisible water vapor. Evaporation can happen at room temperature.

It is still water the whole time. Only its form changes.

What matters here: Water can be ice, liquid water, or water vapor, depending on how hot or cold it is.

How matter changes form

What makes matter change form? Heat.

• Adding heat can melt a solid into a liquid, like ice into water.
• More heat can boil a liquid into a gas, like water into steam.
• Taking heat away can freeze a liquid into a solid, like water into ice.

Worth holding on to: Adding or taking away heat can change matter from one form to another.

A safe home experiment: melt an ice cube

You can watch matter change form. Ask a grown-up for an ice cube.

1. Put an ice cube (a solid) on a plate.
2. Leave it on the counter and watch.
3. Soon it melts into a puddle of water (a liquid).
4. If you leave the puddle for many hours, some of it dries up into the air as gas.

The point: One ice cube can show you a solid, a liquid, and even a gas.

What the tiny bits are doing

Everything is built from bits far too small to see. What makes a solid, a liquid, and a gas different is how those tiny bits behave.

Picture a room full of children.

• Solid: the children stand shoulder to shoulder in neat rows. They can wiggle a little, but nobody swaps places. That is why a solid keeps its shape.
• Liquid: the children are still touching, but now they slide past each other and shuffle around. That is why a liquid can pour and change shape.
• Gas: the children run in every direction with big spaces between them. That is why a gas spreads out to fill a whole room.

Heating can increase particle motion. During melting or boiling, however, added energy can change how particles are arranged and separated without raising the temperature. The children are only a model: particles are not people, and not all solids form neat rows.

The upshot: The bits vibrate around positions in a solid, sliding in a liquid, and flying apart in a gas.

Sort your kitchen

Walk into a kitchen with a grown-up and name the state of each thing.

• An apple: solid.
• Milk in the carton: liquid.
• The air inside the empty carton: gas.
• Honey: liquid, even though it pours very slowly.
• A sponge: solid, even though it is soft and full of holes.
• Steam over a warm pot: gas mixed with tiny water droplets. Stay well back from anything hot, and let a grown-up handle the stove.

Honey trips people up. Slow pouring still counts as pouring, so honey is a liquid.

Sand and sugar trip people up too. A pile of sugar seems to pour like a liquid, but each single grain is a tiny solid keeping its own shape.

Key idea: Pouring slowly still counts as a liquid, and a pile of tiny solids is not a liquid.

Feel trapped air resist a squeeze

With a grown-up, use an undamaged empty plastic drink bottle. Leave its ordinary air inside and close the cap. Gently squeeze the sides. Then have the adult remove and hold the cap while you squeeze again.

The capped bottle is harder to squeeze because you compress the trapped air, raising its pressure. With the cap off, air can leave through the opening. Do not crush or heat the bottle, and keep caps away from young children.

This comparison shows that air occupies space and exerts pressure. It does not measure the air's mass.

What matters here: Invisible air can exert a force on a container.

Common misconceptions

• Thinking gas is not real because you cannot see it. Air is a gas and it is all around you.
• Thinking a liquid changes its amount when it changes shape. It only changes shape.
• Thinking melted ice is a new thing. It is still water, just in liquid form.
• Calling sand or sugar a liquid because it pours. Each grain is a tiny solid.
• Thinking only hard things are solids. A soft pillow is a solid too.

The takeaway

• Matter is anything that takes up space and has mass.
• A solid keeps its own shape.
• A liquid flows and takes the shape of its container.
• A gas spreads out to fill its space.
• The tiny bits are packed in a solid, sliding in a liquid, and far apart in a gas.
• Heat can change matter from one form to another.

Sources

1. OpenStax. Physics, 11.3 Phase Change and Latent Heat. Phase change and latent heat discussion.
2. DOE Office of Science. DOE Explains...Plasma. Opening paragraphs on free electrons and ions.
3. USGS. Glossary of Water Cycle Terms. Evaporation, condensation, groundwater and water vapor entries.

Why a rolling ball stops

Roll a ball across a carpet. It slows down and stops after a couple of metres. Now roll the same ball, the same hard push, across a smooth wooden floor. This time it goes much further before it stops.

Nothing pushed the ball backwards that you could see. No hand, no wall, no wind. Something slowed it down anyway, and it was stronger on the carpet than on the wood. That invisible something has a name, and it belongs to a whole family of pushes and pulls called forces.

What is a force?

A force is a push or a pull. A force can make something start to move, stop, speed up, slow down, or change direction.

When you push a swing, that is a force. When you pull a wagon, that is a force too.

Why this matters: A force is a push or a pull that can make things move or stop.

Pushes and pulls

Push and pull describe everyday forces.

• A push moves something away from you. You push a shopping cart or push a door shut.
• A pull moves something toward you. You pull a drawer open or pull a rope in tug of war.

Look around and you will see pushes and pulls everywhere.

In short: A push moves things away and a pull moves things toward you.

Big force and small force

Does it matter how hard you push? Forces can be big or small: for the same object and other conditions, a larger unbalanced force produces a larger change in motion.

A gentle tap sends a ball a short way. A hard kick sends it far. Compare the same ball with a push lasting the same time and pointing the same way.

The core of it: A bigger force makes a bigger change in how something moves.

Gravity: the pull of the Earth

Here is a force you feel all the time. Gravity is the pull that brings things down toward the Earth.

When you drop a ball, gravity pulls it to the floor. When you jump, gravity brings you back down.

Gravity is why things fall. It keeps you on the ground instead of floating away. It even holds the Moon near the Earth.

Remember: Gravity is the pull that brings things down toward the Earth.

Friction: sliding and grip

Friction opposes sliding, or attempted sliding, between touching surfaces. It can slow a sliding object or help a shoe grip the floor.

Roll a ball on grass and it stops quickly, because grass has a lot of friction. Roll it on smooth ice and it slides far, because ice has little friction.

Friction is helpful. It lets your shoes grip the floor so you do not slip. It helps a bike stop when you use the brakes.

Bottom line: Friction opposes sliding between surfaces. It can slow a sliding toy or help your foot grip the floor as you move forward.

Magnets: a special pull

A magnet is an object that can pull certain metals toward it without even touching them. Magnets pull on things made of iron and steel.

A magnet can hold a paper on the fridge. Magnets have two ends, called poles. Two magnets can pull together or push apart.

Why this matters: A magnet can pull certain metals toward it.

A safe home experiment: ramp race

You can see forces at work with a ramp. Ask a grown-up if you can use a book and a toy car.

1. Lean a book on a pillow to make a small ramp.
2. Let a toy car roll down. Gravity pulls it down the ramp.
3. Now make the ramp steeper. Does the car go faster?
4. Try rolling it onto a rug and onto a smooth floor. Friction stops it faster on the rug.

In short: Gravity pulls the car down, and friction slows it to a stop.

Direction changes everything

A force has a size, and it also has a direction. The direction decides what happens.

Picture a ball rolling toward you.

• Push it from behind, in the same direction it is going: it speeds up.
• Push it from the front, against its motion: it slows down or stops.
• Push it from the side: it turns and rolls a new way.

This is exactly what happens in sports. A soccer player can speed a ball up, stop it, or bend it around a defender, all with the direction of the kick.

Try it yourself with a ball on the floor. Give it three different pushes and watch how the direction of your push changes what it does.

The core of it: The direction of a force decides whether something speeds up, slows down, or turns.

When forces are balanced

Often two forces push on the same thing at once. What happens depends on whether they match.

Think of tug of war. If both teams pull equally hard, the rope stays still. If the rope starts at rest, these balanced forces leave it at rest. Balanced forces can also leave an already moving object travelling steadily in a straight line.

Now one team pulls harder. The forces are unbalanced, and the rope moves toward the stronger team.

A book sitting on a table is balanced too. Gravity pulls it down, and the table pushes up just as hard. That is why it stays put instead of sinking through.

Remember: Balanced forces mean no change in motion. Unbalanced forces change motion.

Test friction on four surfaces

Here is a fair test you can run with one toy car and one ramp.

1. Prop a book on a couple of other books to make a ramp. Keep the same height every time.
2. Let the car roll down and onto a smooth floor. Mark where it stops and measure the distance.
3. Repeat onto a rug, then onto a towel, then onto a piece of aluminum foil.
4. Write each distance in a chart.

Compare the measured distances rather than assuming their order. Soft materials and bumps may slow the wheels, while different surfaces resist motion by different amounts.

What is the variable here? Only the surface. Keep the ramp height, the car, and the starting spot exactly the same.

Bottom line: In this car test, soft or rough surfaces may stop the car sooner. Surface material, wheel behavior, and bumps affect the result.

Push or pull?

Name each one as a push, a pull, or both. Then check.

• Opening a drawer: pull.
• Closing a drawer: push.
• Pedaling a bike: push, with your feet on the pedals.
• A magnet grabbing a paperclip: pull.
• Zipping up a coat: pull.
• Swinging on a swing: both. Someone pushes you, and gravity pulls you back down.

Why this matters: Almost everything you do all day is a push, a pull, or a mix of both.

Common misconceptions

• Thinking motion needs a continuing push. An object can keep moving with balanced forces; unbalanced forces change its motion.
• Thinking heavy things fall faster than light things. Gravity pulls on all of them.
• Thinking friction is bad. Friction helps you walk and stop.
• Forgetting that a still object can still have forces on it, if they are balanced.
• Thinking magnets pull everything. They only pull certain metals like iron and steel.

Pulling it together

• A force is a push or a pull.
• A push moves things away. A pull moves things toward you.
• A bigger force makes a bigger change in motion, and the direction decides what changes.
• Balanced forces do not change motion. Unbalanced forces change speed or direction.
• Gravity pulls things down toward the Earth.
• Friction opposes sliding between surfaces and can help objects grip.

Sources

1. NASA Glenn. Newton's Laws of Motion. First and second laws, net external force and constant-mass acceleration.
2. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

Put your fingers on your throat and hum

Put two fingers on the front of your throat and hum a long note. Feel that? A buzz, right under your fingers.

That buzz is two small flaps inside your throat shaking very fast. The shaking pushes on the air. The air passes the shake along to the next bit of air, and the next, all the way across the room, until it reaches somebody's eardrum and shakes that too. Sound is not a thing that flies through the air. It is a shake being passed along.

What is energy?

Energy is the ability to cause changes or do work. Scientists use 'power' for how quickly energy is transferred, so energy and power are different. Energy can make things move, glow, get warm, or make sound.

You have energy from the food you eat. A car gets energy from fuel. A lamp gets energy from electricity.

Most of the energy on Earth comes from one big source: the Sun.

What matters here: Energy can be transferred to make things happen.

Light energy

Light is a kind of energy that lets us see. Without light, everything would be dark.

The Sun is our biggest source of light. We also get light from lamps, flashlights, and fire.

Light travels in straight lines. When light hits something it cannot pass through, it makes a dark spot behind it called a shadow. A shadow is the dark shape made when something blocks light.

Worth holding on to: Light is energy that lets us see, and it makes shadows when it is blocked.

Heat energy

Heat is energy transferred because of a temperature difference. On its own, this transfer goes from warmer to cooler things. The Sun gives us heat. So does a stove, a fire, and even your own body.

Heat moves from warm things to cold things. That is why a warm cup of cocoa slowly cools down, and why an ice cube in your hand melts.

Heat can also change matter, like melting ice into water or cooking an egg.

The point: Heat is energy transferred because of a temperature difference. It can change temperature or state.

Sound energy

Sound is a kind of energy that we hear with our ears. Every sound is made by something moving back and forth very fast. That fast back-and-forth movement is called a vibration.

When you pluck a guitar string, it vibrates and makes a sound. When you talk, parts inside your throat vibrate.

Sound travels through the air to reach your ears. Big vibrations make loud sounds. Small vibrations make soft sounds.

The upshot: Sound is energy made by vibrations, and we hear it with our ears.

Energy can change form

Can energy change from one kind into another? It can, and it happens all around your house.

• A lamp changes electricity into light and heating.
• A fire releases chemical energy stored in wood as light and heating. The wood's matter also changes into gases and ash.
• A radio changes electricity into sound.
• Your body changes food into the energy to run and play.

Key idea: Energy can change from one form into another.

Where energy comes from

We use energy every day. It comes from many places.

• The Sun gives light and heat.
• Food gives energy to living things.
• Electricity powers lights and machines in our homes.
• Wind and moving water can make electricity too.

What matters here: Energy comes from the Sun, food, electricity, wind, and water.

A safe home experiment: feel the vibration

You can feel sound energy. Try this.

1. Gently rest your fingers on the front of your throat.
2. Hum a long note. Can you feel the buzzing?
3. That buzzing is a vibration making sound.
4. Stop humming and compare how your throat feels. Do not press hard or stretch rubber bands near a face.

Worth holding on to: Sound comes from vibrations you can sometimes feel and see.

Investigate your shadow

Shadows show you that light travels in straight lines. Try this outside on a sunny day, or inside with a flashlight.

1. Stand outside in the morning and have someone trace or mark where your shadow ends.
2. Come back at lunchtime and mark it again.
3. Come back in the late afternoon and mark it once more.

Your shadow is long in the morning, short around midday, and long again in the afternoon. It also swings around to a different side.

Why? Because the Sun appears low in the sky in the morning, high at midday, and low again later. The lower the light, the longer the shadow.

You can copy this indoors with a flashlight and a toy. Hold the flashlight low, then high, and watch the shadow change. Never look straight at the Sun or shine a light in anyone's eyes.

The point: Low light makes long shadows, and high light makes short ones.

Sound travels through solids too

Sound does not only travel through air. It moves through liquids and solids as well, and often better.

Try the table test. Put your ear flat against a table while a friend gently taps the other end with a fingertip. Now lift your head and have them tap just as gently. Compare what you hear, keeping taps gentle and away from ears. Do not bang the table.

Sound can travel through wood as well as air. Its speed and how much weakens along the way depend on the material and setup; closer particles alone do not guarantee a stronger sound.

Whales use this. Sound carries so well in water that whale calls can travel for many kilometres under the sea.

The upshot: Sound travels through solids and liquids, often better than through air.

Follow an energy chain

Energy hardly ever changes just once. It usually goes through a chain of changes.

Follow a flashlight.

1. Chemicals inside the battery store energy.
2. The battery turns that into electrical energy.
3. The bulb turns electrical energy into light energy, plus heating.

Now follow yourself riding a bike.

1. The Sun's energy grew the plants.
2. You ate food that came from those plants.
3. Your muscles turned that food energy into movement.
4. The brakes turn movement into heat when you stop. A brake can become hot enough to burn; do not touch it to test this.

Key idea: Energy passes along a chain, changing form at each step, and most chains start at the Sun.

Watch heat move

Heat on its own moves from the warmer thing to the cooler thing. Here is a safe way to watch it.

1. Ask a grown-up to fill matching cups with equal amounts of room-temperature and refrigerator-cold water. Do not heat the water.
2. Put one ice cube in each cup at the same moment.
3. Watch which one shrinks faster.

Predict which cube will melt sooner, then record what happens. With other conditions alike, the room-temperature water usually transfers energy to the ice faster than the colder water.

Notice what is really happening. The cold is not moving into the water. The heat is moving out of the water and into the ice. That is the direction of net heat transfer on its own. Machines such as refrigerators use electrical energy to move energy from a cooler space to warmer surroundings.

What matters here: Heat on its own flows from warmer things toward cooler things.

Common misconceptions

• Thinking you can see in the dark. Your eyes need light to see.
• Thinking cold moves into warm things. Really, heat moves out of warm things.
• Thinking sound can happen without movement. Every sound comes from a vibration.
• Thinking sound only travels through air. It travels through water and solids too.
• Thinking energy disappears. It usually just changes into another form.

Summing up

• Energy can be transferred to make things happen.
• Light is energy that lets us see and makes shadows.
• Heat is energy transferred because of a temperature difference; it can change temperature or state.
• Sound is energy made by vibrations, and it travels through solids and liquids too.
• Energy can change from one form to another along a chain.

Sources

1. OpenStax. University Physics Volume 2, 1.4 Heat Transfer, Specific Heat, and Calorimetry. Internal Energy and Heat.
2. National Weather Service. Lightning Safety Tips and Resources. When Thunder Roars, Go Indoors; indoor safety and 30-minute rule.
3. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.

Why your shadow is short at lunchtime

Stand outside on a sunny morning and look down at your shadow. It is long, and it stretches away to one side. Go out again at lunchtime and it has shrunk to a stubby puddle around your feet. Go out before dinner and it is long again, pointing the other way.

You did not change size three times. What changed is how high the Sun climbed in the sky. Hold on to that idea, because the very same thing, how high the Sun gets, is what makes summer hot and winter cold.

What is weather?

Weather is what the sky and air are like at one place and time. Weather can be sunny, cloudy, rainy, windy, snowy, or foggy.

Weather can change from morning to afternoon. It can be different in two towns on the same day.

Bottom line: Weather is what the sky and air are like right now.

Weather and temperature

One big part of weather is how warm or cold it is. We call this the temperature. Temperature is a measure of how hot or cold something is.

We use a tool called a thermometer to measure temperature. On a hot day, the temperature is high. On a cold day, it is low.

Why this matters: Temperature tells us how hot or cold the air is.

Clouds and rain

Clouds contain tiny liquid water droplets, ice crystals, or both, suspended in air. When the drops join and grow big and heavy, they fall as rain.

Snow grows as ice crystals in clouds. Hail grows when droplets freeze onto ice in a thunderstorm; upward-moving air helps hold it aloft while it grows. Hail can fall on a warm day.

Rain, snow, and hail are all called precipitation. Precipitation is water that falls from the sky.

In short: Rain, snow, and hail are precipitation, which is water falling from the sky.

Wind

Wind is moving air. You cannot see wind, but you can feel it and see what it does.

Wind can be a gentle breeze that cools your face. It can also be a strong gust that bends the trees.

Wind pushes clouds across the sky, flies kites, and turns pinwheels.

The core of it: Wind is moving air that we can feel but not see.

The four seasons

Weather also changes with the seasons. A season is a time of year with its own kind of weather. Many places describe the year using four seasons. Near the equator, wet and dry seasons may be more noticeable.

• Spring: It gets warmer, rain falls, and plants start to grow.
• Summer: It is hot and often sunny.
• Fall: It gets cooler and many leaves change color and drop.
• Winter: It is cold, and in some places it snows.

The seasons happen because the Earth is tilted as it travels around the Sun.

Remember: The four seasons are spring, summer, fall, and winter, each with its own weather.

Weather and climate are different

Weather is what happens today. Climate is the usual weather of a place over many years.

A rainy Tuesday is weather. The fact that a desert is usually dry all year is its climate.

Bottom line: Weather is today. Climate is the usual weather over many years.

Why weather matters

Weather helps us decide what to do. We check it to know what to wear and whether to bring an umbrella.

Farmers watch weather to know when to plant. Pilots check it to fly safely. Weather forecasters use tools to guess tomorrow's weather.

Why this matters: People watch weather to stay safe and plan their day.

A safe home activity: be a weather watcher

You can track weather like a scientist. Try this for a week.

1. Each morning, look out the window.
2. Draw a picture of the sky: sun, clouds, or rain.
3. Is it warm or cold? Windy or still?
4. After a week, look back. What days were alike? What changed?

In short: Watching the sky each day helps you learn how weather changes.

Why the seasons happen

Many people think summer comes because Earth moves closer to the Sun. That is not it. The real reason is the tilt.

Earth spins on a line through its middle, and that line is tipped over a little, like a spinning top leaning to one side. The tilt stays pointing the same way all year as Earth travels around the Sun.

Around summer, your hemisphere, the northern or southern half of Earth, leans toward the Sun. Sunlight arrives more directly and days are longer, so it feels hot. That is summer.

Around winter, your hemisphere leans away. Sunlight arrives at a slant and days are shorter, so it feels cold. That is winter.

Here is proof that distance is not the answer. Northern and Southern Hemispheres have opposite summer and winter seasons. This means north and south of the equator, not simply places on opposite sides east and west. Earth cannot be near and far at the same time, but it can be tilted toward one half and away from the other.

The core of it: Seasons come from Earth's tilt, not from being closer to the Sun.

Measure rain without cutting a bottle

With a grown-up, choose a clear plastic container with a flat bottom and straight vertical sides. Its opening must have the same width as the measuring part; do not add a funnel or stones inside.

1. Stand the empty container upright on a level, secure surface away from roofs and trees.
2. After rain, and only when the weather is safe, measure the water depth from the inside bottom using a ruler.
3. Record the date, collection time, and depth in millimeters. Empty the container afterward.

A 5-millimeter depth means that much rain collected over the container's opening. A wider funnel would catch extra water and make an ordinary ruler read too high. Wind, splashes, and evaporation can affect a homemade gauge.

Remember: Matching the opening and collecting area helps turn water depth into a useful rainfall estimate.

Read a forecast like a scientist

A forecast is a careful guess about tomorrow's weather, based on measurements taken today.

Forecasters use tools you now know about: thermometers for temperature, rain gauges for precipitation, and instruments that measure wind. Satellites in space watch the clouds move.

A forecast often says something like a 40 percent chance of rain. That does not mean the forecaster is unsure of everything. For a National Weather Service forecast, it means a 4-in-10 chance of measurable precipitation at the forecast location during the stated time. It does not mean rain for four-tenths of the day, or a count of ten identical past days.

Try this. Write down tomorrow's forecast, then check the next day whether it came true. Do it for a week and count. One week cannot tell you whether a probability forecast is well calibrated. Across many 40-percent forecasts, rain should occur about 40 percent of the time if those probabilities are well calibrated.

Bottom line: A forecast is a measured guess, and checking it is real science.

Weather safety

Knowing the weather helps you stay comfortable and safe.

• On very hot days, drink water often, wear a hat, and find shade.
• On very cold days, wear layers and cover your hands, ears, and head.
• If you hear thunder, seek a substantial building or enclosed metal-topped vehicle immediately with a grown-up. Wait at least 30 minutes after the last thunder; stay away from windows, plumbing, and plugged-in equipment.
• Always follow what your grown-ups tell you when the weather turns rough. They are watching the forecast for you.

Why this matters: Checking the weather helps you dress right and stay safe.

Common misconceptions

• Mixing up weather and climate. Weather is today. Climate is over many years.
• Thinking clouds are made of cotton or smoke. They contain tiny liquid droplets, ice crystals, or both.
• Thinking summer comes because Earth is closer to the Sun. It comes from the tilt.
• Thinking wind is not real because you cannot see it. Wind is moving air you can feel.
• Thinking it is the same weather everywhere. It can differ from town to town.

Looking back

• Weather is what the sky and air are like right now.
• Temperature tells us how hot or cold it is.
• Rain, snow, and hail are precipitation.
• Wind is moving air.
• The four seasons come from Earth's tilt as it circles the Sun.
• A forecast is a measured guess you can check for yourself.

Sources

1. NASA Space Place. What Causes the Seasons?. Tilt and opposite hemispheres explanation.
2. National Weather Service Louisville. What Does Probability of Precipitation Mean?. Definition and 30 percent example (0.01 inch at a point over a time period).
3. National Weather Service Green Bay. Types of Winter Weather. Snow and sleet descriptions.
4. NOAA National Ocean Service. Build Your Own Weather Station. Build a Rain Gauge: straight-sided container.
5. National Weather Service. Lightning Safety Tips and Resources. When Thunder Roars, Go Indoors; indoor safety and 30-minute rule.

The puddle that vanished

It rained on Tuesday and there was a puddle on the path outside. By Thursday the puddle was gone.

Nobody drank it. Nobody mopped it up. Some water might soak into pores or cracks in concrete, and some might evaporate. So where did a whole puddle go?

Water that evaporated entered the air as invisible vapor. Water that soaked in followed a different path. Water on Earth almost never disappears. It just moves, round and round, on a journey that has been running for billions of years.

Water is always moving

The water on Earth is always on the move. It goes from the oceans and lakes, up into the sky, and back down again.

This journey happens again and again, so we call it the water cycle. A cycle is something that repeats over and over.

The amazing part is that the Earth reuses the same water. The rain today may have been in an ocean last week.

Key idea: The water cycle is the never-ending journey water takes from the ground to the sky and back.

Step 1: Evaporation

The Sun warms the water in oceans, lakes, and puddles. When water gets warm, it turns into a gas called water vapor and floats up into the air. This is called evaporation.

Evaporation is a change from liquid water to water vapor at the liquid's surface. It can happen in shade or at night too. You cannot see water vapor, but it is there.

This is why a wet sidewalk dries up on a sunny day. The water did not vanish. It evaporated into the air.

What matters here: Evaporation changes liquid water at a surface into invisible water vapor.

Step 2: Condensation

High up in the sky, the air is cold. The cold air makes the water vapor turn back into tiny water drops. This is called condensation.

Condensation changes vapor into liquid drops when conditions allow. Many clouds also contain ice crystals; cooling moist air can help form them.

You can see condensation at home. When you take a cold drink out on a warm day, drops form on the outside of the glass.

Worth holding on to: Condensation is when water vapor cools and turns back into drops, making clouds.

Step 3: Precipitation

The tiny drops in a cloud bump into each other and grow bigger. When they get too big and heavy to float, they fall down. This is called precipitation.

Precipitation is water falling from clouds, including rain, snow, sleet, and hail. Snow forms from ice crystals; hail grows in thunderstorms and can fall in warm weather.

The point: Precipitation is when the drops grow heavy and fall from the sky as rain or snow.

Step 4: Collection

When the water falls, it lands on the ground. It flows into rivers, lakes, and oceans, or it soaks into the soil. This is called collection.

Then the Sun warms this water again, and the whole cycle starts over. Around and around it goes.

The upshot: Collection is when fallen water gathers in oceans, lakes, and rivers, ready to start again.

The cycle goes round and round

Let us put the four steps together:

1. Evaporation: The Sun warms water and it rises as gas.
2. Condensation: The gas cools and makes clouds.
3. Precipitation: Water falls as rain or snow.
4. Collection: Water gathers in oceans and lakes, and it starts again.

This cycle has been going for millions of years. It gives us the fresh water we drink.

Key idea: These four processes form one useful route through the water cycle. Water can take other routes or remain stored for a long time.

Watch condensation on a cold cup

With a grown-up, dry the outside of a clear plastic cup and fill it with cold water and ice. Set it on a dry tray. Do not heat any water or taste the experiment materials.

In humid air, droplets may appear outside the cup. They come from water vapor in the surrounding air condensing on the cold surface, not water leaking through the cup. In very dry air, few droplets may appear.

What matters here: Condensation can form droplets on a cold surface. Clouds involve droplets or ice suspended in air, so a wet cup is not itself a cloud.

The Sun is the engine

Something has to lift all that water into the sky. Water does not climb up on its own.

The Sun does the lifting. Sunlight warms the ocean, and warming gives the water the energy to change into vapor and rise. The Sun supplies most of the energy driving Earth's water cycle. Evaporation does not require sunlight at that moment: water can take energy from its surroundings at night too.

You can see the Sun doing the work. Spill a little water on a sunny sidewalk and on a shady sidewalk at the same time. The sunny puddle disappears much faster.

So the water cycle is really a solar-powered machine. Solar energy helps water evaporate, moving air transports vapor, and gravity pulls precipitation and flowing water downhill.

Worth holding on to: The Sun gives water the energy to rise, so it powers the whole cycle.

Where Earth's water is stored

The USGS reproduces a global water estimate published in 1993. In that estimate, about 96.5 percent of Earth's total water is in oceans, seas, and bays. About 1.7 percent is in ice caps, glaciers, and permanent snow, and about 0.8 percent is fresh groundwater.

Picture 100 equal cups representing all Earth's water: roughly 96 and a half cups represent oceans, fewer than two cups represent this ice, and less than one cup represents fresh groundwater. These rounded amounts do not list every store of water.

The denominator matters. Within fresh water alone, the same estimate puts about 69 percent in ice and 30 percent underground. 'Fresh' means low in dissolved salt; it does not mean safe to drink or easy to reach. Water may need treatment before use.

The point: Most water is salty, and much fresh water is frozen or underground. Available clean water depends on location as well as the total amount.

Race two dishes

Here is a fair test of evaporation you can set up in five minutes.

1. Pour the same small amount of water into two identical shallow dishes.
2. Put one on a sunny windowsill and one in a cool, shady spot.
3. Mark the starting water level on the outside of each dish.
4. Check every few hours and mark the new level.

The sunny dish loses water much faster. Warmth speeds evaporation up.

Want a second test? Pour the same amount into one wide dish and one narrow glass. The wide one dries faster, because more water surface is touching the air.

The upshot: Water evaporates faster when it is warmer and when more of it touches the air.

Water under the ground

Not all the rain runs into rivers. Some of it soaks down into the soil and collects between grains of sand and cracks in rock. That hidden water is called groundwater.

Many plants take water held in soil; some roots also reach groundwater below the water table. People pump it up through wells. In many towns, the water that comes out of the tap started as rain that soaked into the ground long ago.

Plants also send water back up into the sky. Water travels from the roots to the leaves and slips out of tiny holes as vapor. The amount varies with the tree and its growing conditions.

Key idea: Water also travels underground and back up into the sky through plants.

Common misconceptions

• Thinking rain is brand new water. It is the same water, reused over and over.
• Thinking a puddle that dries up just vanished. It evaporated into the air.
• Mixing up the words. Evaporation goes up. Precipitation comes down.
• Thinking all of Earth's water is drinkable. Almost all of it is salty or frozen.
• Thinking clouds are solid. They contain tiny liquid droplets, ice crystals, or both.

What you now know

• The water cycle is the never-ending journey of water.
• Evaporation: the Sun warms water and it rises as gas.
• Condensation: the gas cools and forms clouds.
• Precipitation: water falls as rain or snow.
• Collection: water gathers, and the cycle starts again.
• The Sun powers the whole cycle, and fresh water is only a small slice of it.

Sources

1. USGS. Glossary of Water Cycle Terms. Evaporation, condensation, groundwater and water vapor entries.
2. USGS Water Science School. Where Is Earth's Water?. Global water-distribution table and Shiklomanov/Gleick 1993 source note.
3. National Weather Service Green Bay. Types of Winter Weather. Snow and sleet descriptions.

Three centimeters to Earth, ninety to Neptune

Draw the Sun at one end of a long paper strip. On a distance model where Earth is 3 centimeters from the Sun, Neptune belongs about 90 centimeters away.

The inner planets cluster near the Sun while the outer planets are much farther apart. The paper model later in this lesson uses one scale for every distance. Its planet symbols are not sized to that scale.

What is the solar system?

The solar system is the Sun and everything that travels around it. That includes eight planets, their moons, and many smaller rocks.

The planets orbit the Sun. Moons orbit planets while travelling with them around the Sun; their paths are not all perfect circles.

Remember: The solar system is the Sun and everything that travels around it.

The Sun is a star

The Sun is a star. A star is a huge ball of hot, glowing gas. The Sun looks small because it is very far away, but it is really enormous.

The Sun gives us light and heat. Without it, Earth would be dark, frozen, and empty.

Never look straight at the Sun. Its light is so strong it can hurt your eyes.

Bottom line: The Sun is a star that gives Earth light and heat.

What is a planet?

A planet is a large, round object that travels around the Sun. The path a planet takes around the Sun is called its orbit.

There are eight planets. In order from the Sun, they are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Why this matters: A planet is a large, round object that orbits, or travels around, the Sun.

Our home planet: Earth

Earth is the third planet from the Sun, and it is our home. As far as we know, it is the only planet with living things on it.

Earth is just the right distance from the Sun. It is not too hot and not too cold. It has air to breathe and water to drink.

In short: Earth is our home planet, with air, water, and life.

The Moon

The Moon is a big ball of rock that travels around the Earth. It is our closest neighbor in space.

The Moon does not make its own light. It shines because sunlight bounces off it.

The Moon seems to change shape through the month. These shapes are called phases. Really the Moon is round the whole time. We just see different amounts of its lit side.

The core of it: The Moon is a ball of rock that circles Earth and reflects the Sun's light.

Stars and space

At night, you can see tiny points of light in the sky. Most of those are stars, just like our Sun, but so far away they look tiny.

There are more stars than anyone can count. Our Sun is only one of them.

Big groups of stars make up a galaxy. Our galaxy is called the Milky Way.

Remember: Stars are faraway suns, and huge groups of them form galaxies.

Day and night

Why do we have day and night? The Earth spins around like a top. This spinning is called rotation.

As the Earth spins, one side faces the Sun and has day. The other side faces away and has night. The Sun returns to about the same position in our sky after roughly 24 hours, one solar day.

Bottom line: Day and night happen because the Earth spins around.

A safe home activity: be the Earth

You can act out day and night. Ask a grown-up to help.

1. Put a lamp in the middle of a room. The lamp is the Sun.
2. You are the Earth. Stand facing the lamp. Your face is in daytime.
3. Slowly spin around in place.
4. When your face turns away from the lamp, that is nighttime.

Why this matters: Spinning shows how one Earth can have day on one side and night on the other.

Two families of planets

The planets have different compositions.

The first four, Mercury, Venus, Earth, and Mars, are the rocky planets. They are smaller, they have solid ground you could stand on, and they sit closer to the Sun.

Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants. These outer planets have deep atmospheres and no hard surface like Earth's to land on. 'Ice giant' describes their composition, not a frozen surface for walking.

Jupiter is so big that more than a thousand Earths could fit inside it. Saturn has bright rings made of countless chunks of ice and rock.

Here is a sentence some people use to remember the order: My Very Excellent Mother Just Served Us Noodles. The first letter of each word matches a planet, in order from the Sun.

In short: The inner planets are rocky; the outer planets include two gas giants and two ice giants.

Measure a paper solar system

With a grown-up, tape paper into a strip at least 91 centimeters long. Mark the Sun at zero. Use these rounded distances from the Sun, not gaps between neighboring planets:

NASA's reference table gives distances in astronomical units, with Earth's distance represented by 1. Our model uses 3 centimeters per unit: Neptune's 30.06 units times 3 gives 90.18 centimeters, rounded to 90.2. These are approximate orbital distances, not today's exact positions.

Draw small labeled marks rather than pretending the planets are the right size. Planets are not usually lined up like this either. A model can show distances without showing every feature.

The core of it: Keep one distance scale, and say what the model leaves out.

Why the Moon changes shape

Here is the activity that makes Moon phases click. Use a cool battery-powered light and a ball, with a grown-up. Do not remove lamp shades, touch bulbs, or use a dark room where someone could trip.

1. The lamp is the Sun. Your head is the Earth. The ball is the Moon.
2. Hold the ball out at arm's length and slowly turn all the way around in a circle.
3. Watch the lit part as you turn slowly. Hold the ball slightly above your head's shadow, or you will model an eclipse instead of an ordinary full moon.

When the ball is on the far side of you from the lamp, you see its whole lit face. That is a full moon.

When the ball is between you and the lamp, you see only its dark side. That is a new moon.

In between, you see a curved sliver or a half. Those are the crescent and quarter phases.

The ball never changed shape once. Only your view of the lit half changed. The real Moon works exactly the same way, taking about a month to go all the way around.

Remember: Moon phases are just different views of the same lit half.

Why we do not feel Earth spinning

Earth spins fast, and it races around the Sun even faster. So why does nothing feel like it is moving?

Because you and your nearby surroundings share Earth's overall motion. You do not slide past the ground simply because Earth rotates.

It is like riding in a smooth car. If you close your eyes on a straight highway, you barely feel that you are moving at all. You only notice motion when something speeds up, slows down, or turns.

Earth's rotation has measurable effects, but they are small compared with the everyday forces we notice. Sharing its motion explains why we do not feel a constant rush past the ground.

Bottom line: We do not feel Earth move because everything around us moves with us.

Common misconceptions

• Thinking the Sun moves around the Earth. Really, the Earth spins and orbits the Sun.
• Thinking the Moon makes its own light. It reflects sunlight.
• Thinking stars are small. They are huge suns that are very far away.
• Thinking the planets are evenly spaced. The outer ones are much farther apart.
• Thinking the Moon changes its real shape. We just see different parts lit up.

Recap

• The solar system is the Sun and everything that orbits it.
• The Sun is a star that gives us light and heat.
• There are eight planets: four inner rocky planets, two gas giants, and two ice giants farther out.
• The Moon circles Earth and reflects sunlight.
• Moon phases come from seeing different amounts of the lit half.
• Day and night happen because the Earth spins.

Sources

1. NASA JPL. Solar System Size and Distance Reference Guide. One-page table: planet distance from Sun in AU.
2. NASA Science. Planets. Planet overview and gas/ice giant classification.
3. NASA Space Place. What Causes the Seasons?. Tilt and opposite hemispheres explanation.

What is actually in your bin

Before you read any further, write down a guess: if you recorded the items your family is about to discard, before they enter a bin, which pile would be biggest? Food? Paper? Plastic? Metal? Glass?

Ask a grown-up to help keep a tally without touching mixed rubbish. Do not empty bins or handle sharp, dirty, or unknown items. Compare your prediction with your own family's count; homes produce different kinds of waste.

Why our planet needs our help

The Earth gives us air, water, food, and a home for all living things. We need to take good care of it.

Sometimes people make too much trash or make the air and water dirty. Dirty air and water are called pollution. Pollution is anything that makes the land, air, or water unclean.

Pollution can hurt plants, animals, and people. The good news is that we can all help stop it.

The upshot: The Earth is our only home, and we can help keep it clean.

The three Rs

There are three easy words that help us take care of the Earth. They all start with the letter R: reduce, reuse, and recycle.

These three Rs help us make less trash and save the Earth's things. Let us look at each one.

Key idea: The three Rs are reduce, reuse, and recycle.

Reduce: use less

To reduce means to use less of something. When we use less, we make less trash.

Here are ways to reduce:

• Turn off lights when you leave a room to use less electricity.
• Turn off the water while you brush your teeth.
• Take only as much food as you will eat.
• Use a lunchbox instead of new bags every day.

What matters here: To reduce is to use less, which makes less trash.

Reuse: use it again

To reuse means to use something again instead of throwing it away.

Here are ways to reuse:

• Use a water bottle you can fill up again and again.
• Turn a clean plastic tub with no sharp edges into a pencil holder.
• Give clothes you have outgrown to a younger child.
• Draw on the back of used paper.

Worth holding on to: To reuse is to use something again instead of throwing it away.

Recycle: make it into something new

To recycle means to turn old things into new things. Old items are collected, broken down, and made into brand new items.

Many things can be recycled:

• Paper can become new paper.
• Glass bottles can become new glass.
• Plastic bottles can become new bottles or even clothes.
• Cans can become new cans.

Many homes have a recycling bin. Putting the right things in it helps them get a new life.

The point: To recycle is to turn old things into new things.

Other ways to help the Earth

The three Rs are a great start. Here are more ways to help:

• Plant a tree or flowers. Plants clean the air and feed animals.
• Pick up litter you see, but only with a grown-up's help.
• Walk or bike for short trips instead of riding in a car.
• Take care of animals and the places where they live.

The upshot: Planting, cleaning up, and using less all help the Earth.

A safe home activity: a reuse project

Turn trash into treasure. Ask a grown-up to help.

1. Find a clean, empty container, like an uncoated cardboard box or a sturdy plastic tub with no sharp edges.
2. Wash it and let it dry.
3. Decorate it with paper, crayons, or stickers.
4. Use it to hold pencils, toys, or treasures. You just reused it.

Key idea: With a little work, old things can become useful again.

Some things run out and some come back

The useful things Earth gives us are called resources. Wood, water, metal, and sunlight are all resources. They come in two kinds.

A renewable resource can be replaced within a person's lifetime.

• Sunlight arrives fresh every single day.
• Wind keeps blowing.
• Trees grow back, as long as we plant new ones and do not cut too fast.

A nonrenewable resource is not replaced as fast as people use it on human timescales. Fuels are consumed when burned, while metals can often be recovered and recycled.

• Coal and oil formed underground over millions of years.
• Metals like copper and aluminum come from ore dug out of the ground.

This is why recycling metal cans matters so much. A recycled can becomes a new can, so we do not have to dig up more ore.

What matters here: Renewable resources come back quickly. Nonrenewable resources are not quickly replenished, though some materials can be recycled.

Reduce is the strongest R

The three Rs are listed in that order for a reason. Reduce comes first because it is the most powerful.

Think about a plastic bottle. Recycling it uses trucks, machines, and energy. Reusing a suitable durable bottle also takes resources for making and cleaning it. Using one you already own can avoid buying repeated single-use bottles.

So the order to try is:

1. First, ask if you need it at all. That is reduce.
2. If you do need it, can you use something you already have? That is reuse.
3. When it is truly finished, put it in the right bin. That is recycle.

Worth holding on to: Not needing something is even better than recycling it.

Do a trash audit

Here is a real investigation. Ask a grown-up first, and do not touch anything sharp or messy. Just look and count.

1. For one day, keep a tally of what your family throws away. Paper, food scraps, plastic, cans, and other.
2. At the end of the day, count each group.
3. Look at your biggest group. Ask: could any of that have been reduced, reused, or recycled?

Your biggest group depends on what your family discards. Some food scraps can be composted where a suitable system accepts them, and some paper can be recycled. Ask a grown-up to check local guidance before choosing a change.

Share what you find with your family, then pick one change to try for a week.

The point: Counting your trash shows you exactly where you can make the biggest change.

What goes in the bin?

Recycling only works if the right things go in. Putting the wrong thing in can spoil a whole load.

Usually yes:

• Clean paper and flattened cardboard.
• Metal food and drink cans, rinsed out.
• Plastic bottles and jugs, rinsed, with the cap on if your program says so.

Usually no:

• Used paper towels. Pizza-box rules vary: many programs accept boxes with grease after food scraps are removed.
• Thin plastic bags and plastic wrap, unless a store collects them.
• Broken glass or dishes.

Rules differ from town to town, so ask a grown-up to check your local list. When you truly do not know, it is better to leave it out than to guess.

The upshot: Rinse it, check your local list, and when in doubt, leave it out.

Common misconceptions

• Thinking one kid is too small to help. Every small action adds up.
• Thinking all trash can be recycled. Check what your bin takes.
• Mixing up the three Rs. Reduce means use less, reuse means use again, recycle means make new.
• Assuming all packaging belongs in one bin. Remove food scraps and check local rules, including for pizza boxes.
• Forgetting to ask a grown-up before picking up litter.

What to carry forward

• The Earth is our only home, and we can keep it clean.
• Pollution is anything that makes the land, air, or water unclean.
• Renewable resources can be replenished; nonrenewable supplies are not quickly replaced. Both need careful use.
• Reduce means to use less, and it is the strongest of the three Rs.
• Reuse means to use something again.
• Recycle means to turn old things into new things.

Sources

1. US EPA. How Do I Recycle Common Recyclables?. Paper/cardboard and local program guidance.
2. US EPA. Recycling Basics and Benefits. Benefits and recycling process sections.
3. NSTA Safety Advisory Board. Safer Remote Instruction and Parent Teaching Guide for Elementary. Pages 1-3, hazard review, adult involvement, safe alternatives.