[intro].

Today we’re celebrating a birthday! [Squeaks squeaks.] Oh, don’t worry, Squeaks! It’s not my birthday.

It’s Sir Isaac Newton’s birthday. Sir Isaac Newton was a famous scientist who lived a long time ago. He was especially interested in how things move, even really big things like planets!

He was also a famous mathematician. A mathematician is a person who uses math to tackle big questions about the world. Newton was born in England almost 400 years ago.

He made lots of important discoveries about the world, including figuring out that light is actually made up of all different colors. He also helped invent a whole new type of math! It’s called calculus, and you can use it to learn about how things change.

Today, lots of people learn it in high school or college. Newton made some amazing discoveries about how things move, too — everything from gigantic planets to, say … an apple. Are you ready for a story?

Because there’s a kind of funny one about Newton and an apple. One day, Sir Isaac Newton was at home, sitting under an apple tree in his family’s garden. But then, an apple fell off the tree.

It might have even bopped him right on the head! And right then and there, Newton started to come up with one of his most important theories: the theory of gravity. Newton thought to himself: “Why did that apple fall from the tree?

Why didn’t it float up in the air?” We’re all pretty used to that just being the way things work: when you throw something into the air, it falls down. But the apple got Newton thinking about why they fall down. And the answer he came up with was a force called gravity.

A force is anything that’s a push or a pull. So if I pull this apple across the table, I’m putting a force on it. The force of gravity pulls apples, and people, and just about everything else, toward the ground.

It’s why when we jump up in the air – – We come right back down! Gravity is also why the moon goes around the Earth. That’s right, the same reason why an apple doesn’t float away is also why the moon doesn’t just float out into space.

Of course, the moon isn’t actually a big apple. But they’re both controlled by the same thing: gravity. Newton discovered that things that are really really heavy attract things that aren’t as heavy.

Attract means when something pulls something else towards itself. So because the Earth is incredibly big with lots of heavy stuff inside it, things like apples, and people, and just about anything else you can think of, won’t float away because the Earth is pulling them down with – there’s that special word again – gravity! And since the Earth is heavier than the moon, the earth pulls the moon towards its center.

But the moon doesn’t just crash straight into the ground! That’s because at the same time as it’s being pulled toward Earth, it’s also moving sideways. It’s kind of like the moon is trying to escape, but since the Earth is pulling on it, it swings around and around in a circle instead.

Gravity is also why the Earth goes around the sun. The sun is /muuuuuch/ heavier than the earth. It’s humongous, and if you could weigh it, it would weigh a whole lot!

So its /gravitational pull/ is a lot stronger. Earth is pulled in and we end up going around and around the sun. All because of gravity!

And Newton figured out all of that when an apple fell off a tree. The only thing is, although that apple story is actually pretty famous, and Newton himself used to even tell it…. It may not be totally true.

But it may not be totally un-true, either! The truth might be… somewhere in between. So we do know that Newton was inspired by an apple tree, because a person named William.

Stukeley wrote a long long time ago about having a conversation with him about it. And in this conversation, Newton mentioned watching how apples from a tree fell to Earth. But Newton never mentioned the apple hitting him on the head.

That probably just got added to the story over time as people kept retelling it. We all love the apple story so much that there are apple trees that grew from that very first apple tree all over the world. There are even special places you can go to visit these apple trees, in lots of different countries.

So did that apple fall from the tree and bonk Newton on the head? Probably not. But did an apple falling from the tree to the ground inspire Newton’s theory of gravity.

Very possibly! Well, Squeaks, to celebrate Newton, what do you say we make an apple pie instead of a birthday cake? Maybe we shouldn’t drop it to see if it falls, though!

That would be much messier than just dropping a regular apple … plus, this way we can actually eat it! Thanks for joining us! If you want to keep learning more about how our extraordinary world works, hit the subscribe button, and we'll see you next time here at the fort. [ OUTRO ].