Squeaks, It’s a great day to be alive!

And yes, I’m really living — and so are you! But what does that mean?

It sounds like a pretty simple question, right? There are living things all around us...as well as things that are not living. When was looking around the Fort’s playground a minute ago, I saw a rock — which is not a living thing.

But the bird singing in the tree over there? That’s definitely living. And the tree is, too!

But sometimes the answer to the question of what’s a living thing, and what’s not, can be a little tricky. I can hear a bird sing when I watch a video on a computer or phone—but, of course, computers and phones aren’t living things … even though they might make sounds exactly like things that are alive. Scientists called biologists study all sorts of living things — so they’re the experts on what things are living, and what things are not.

After studying all kinds of life, they discovered that everything that’s alive, or was once alive, has a few things in common. You can think of them kind of as rules that all living things follow. So let’s take a look at some of these rules!

Rule number one: living things need energy. You need energy to play and to do your school work. A plant needs energy to make new leaves and to grow.

Even some kinds of germs use energy when they make us sick! Living things get energy in lots of different ways, but they all need it to stay alive. Rule number two: Living things grow and change.

You’re bigger than you were than when you were a baby — but you’re not as big as you will be someday! And just like you, all living things grow and change — whether that means growing up, or getting more branches and making new leaves, like trees do. Rule number three: Living things reproduce.

Reproduce means to make more of something. When living things reproduce, they make more of themselves! For animals, that means having babies like kittens and puppies that will grow up to become adult cats and dogs.

For plants, that usually means making seeds that can grow into new plants. Rule number four: Living things respond to the world around them. In other words, when something around a living thing changes, that living thing changes too.

For example, when you get cold, you might shiver or put on a coat. Both of these things will help warm you up. So that’s how you respond to the cold.

And in the same way, in autumn, when the days start to get shorter and the weather gets cooler, some trees lose their leaves. They are responding to the change of seasons! Great question, Squeaks!

What about the leaves that have fallen to the ground? Are they living or not living? Those leaves on the ground...even the brown, dead ones...were part of a living thing...the tree...so biologists say we still call them living things.

Now, it might seem like there’s some things missing from the rules. Like, what about … moving? When you think about it, a lot of living things move.

But not all of them do. Plants, for example, don’t get up and walk around, but they’re still alive! And there are things that move that were never alive, like shadows, or the hands on a watch.

Shadows don’t use energy. And watches don’t reproduce or grow. So, if you’re thinking like a biologist, then something has to meet all of the rules in order for it to be considered a living thing.

If it doesn’t, then it’s not alive. Now, are you ready to test what you know about living things? It’s game time!

I’ll name a thing, and you have to decide whether it’s alive or not. First, is an earthworm living or not living? If you said 'living,' you're right!

Earthworms need energy to wriggle through the soil. They also grow. And they lay eggs that hatch into baby earthworms, which means they reproduce.

And, if you’ve ever held an earthworm up, you know they respond to changes in the world around them. If you pick one up, it tries to wiggle away! Let’s try another one.

Is a cloud living or not living? If you said 'not living,' you're right! A cloud may look like it grows because it might get bigger, say, before it rains.

But a cloud doesn’t use energy, and it doesn’t have baby clouds...so it doesn’t reproduce. And since it doesn’t follow all of the rules...it’s not living! What about this stick?

Is it living or not living? If you said 'living,' you're right! This stick is living because it was once part of a tree.

And, since the tree is a living thing, then the stick can also be called a living thing. You can use these easy rules to study everything you see around you! Go for a walk outside, and take some time to stop and think about what you see -- icicles, acorns, pebbles, and grass -- and look for the clues we talked about: Are they alive or not?

Thanks for joining us on SciShow Kids. If you have a question for us, ask a grownup to help you to leave us a comment down below, or to send an email to Kids@SciShow.com! Thanks, and we'll see you next time here at the Fort!