The world around you is just full of things that are on the move.

Airplanes in the sky, cars on the road and tons of animals -- including you and me! -- are walking around everywhere. But you know what?

It turns out that the ground beneath your feet is moving too! Most of the time, you can’t feel it. Because, most of the time, the ground is moving very slowly.

But when you do feel the ground move, that’s called an earthquake. During an earthquake, the ground shakes—sometimes a little, and sometimes a lot. It might seem kind of strange that the ground, which holds up houses and skyscrapers and everything else, can actually move.

But it does—and it’s all because of the way the Earth is made. If you could cut the Earth in half, and look at it like this, you’d see that the Earth isn’t a solid ball all the way through. It has layers—kind of like a cake!

The top, or outside layer of the Earth...what we think of as the ground...is called the crust. Now, even though the crust of the Earth is certainly very strong—it’s not made of one big piece. The Earth’s crust is actually made of many big pieces that fit together.

You can think of these pieces as being like the pieces of a giant jigsaw puzzle. And we call these pieces of the crust plates. Now, the edges of these plates aren’t smooth along their edges, like the edges of the plates you eat off of These plates are made of very thick layers of rock—so their edges are bumpy and ragged, with rocky chunks sticking out of them.

And it’s these plates that make up the Earth’s crust that are always moving. Like I said earlier, you usually can’t feel or see them moving, because they move very, very slowly. Most plates just creep along at about one or two centimeters a year.

That’s slower than your fingernails grow! But how do these moving plates cause earthquakes? Well, if you look at pieces of a puzzle, you’ll see there there’s a gap between the pieces where they touch.

And, there’s a line where the plates touch, too. We call that line a fault. Some faults are very thin, and too small to be seen.

And some are very deep in the Earth’s crust. But some faults are really big, and you can see them right on the Earth’s surface. For example, this fault, which runs almost the whole length of the state of California, is more than a thousand kilometers long!

And faults are where most earthquakes happen. As the plates of the Earth’s crust move past each other at a fault, the pieces of rock that stick out of their sides sometimes bump into one another. And when this happens, the ground above the plates—and anything on the ground—shakes.

But, sometimes the plates do more than just bump into each other. They get stuck! Have you ever tried to open a door that’s stuck?

You push and push...and then, all of a sudden...it opens really fast and bangs against the wall! Well, sometimes, two different plates can’t move past each other...and they get stuck. But they keep trying to move!

They push against each other, just like you pushing against that stuck door! When two things push against each other, the force of all of that pushing causes what we call pressure. And if the thing that’s being pushed doesn’t move, that pressure has nowhere to go.

So it just keeps building up. In the case of our plates of crust, the pressure builds up where they’re stuck. It builds and builds...until the rocks break...and the plates suddenly move.

This causes the ground above the plates to shake … sometimes a lot. How much the ground shakes depends on how much pressure has built up between the plates. The more pressure...the bigger the earthquake.

So there’s a lot of cool stuff happening beneath your feet! Even if -- most of the time -- you can’t even feel it! Thanks for joining us on SciShow Kids!

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