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Did you know that of all of the rocks in the world, there are only 3 main kinds? What are they? And how can you tell them apart? Jessi and Squeaks show you how you can become a rock detective!

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(SciShow Kids intro plays)

Jessi: Hi guys! Squeaks and I are checking out our rock collections. Oh! That's a nice one! Now, some people might wonder why we collect rocks. I mean, a rock's a rock, right? Well, you've probably noticed that rocks don't all look the same. Some of them have speckles or little bits of what look like glitter and some of them are just one color. And rocks don't feel the same, either. Some are coarse and gritty and some are smooth.

It turns out there are thousands of different rocks, so you could build a really big rock collection if you tried. But among all those thousands of rocks, scientists organize them into three main groups. These groups are called igneous, sedimentary, and metamorphic, and each group is based on how a rock actually forms in nature. That's right, everything comes from somewhere, even rocks.

For example, igneous rocks form when magma, the liquid rock under the surface of the earth, hardens. You know about magma, it can erupt from volcanoes. Once magma comes above ground, it's called lava, and when this hot melted rock cools and hardens, whether it's above ground like on the side of a volcano or deep underground, it becomes igneous rock.

Now, sedimentary rock forms when other rocks break down. This happens a lot actually, all the time, but it happens really slowly. Sedimentary rocks often form in water because they're made of pressed-together parts of other rocks that were broken up and carried away by things like rivers and streams. As a river passes over the land, it wears down the rock beneath it into tiny broken pieces called sediment. Eventually, all that sediment settles to the bottom of the water and builds up, and over time, pressure pushes down on the sediment, squeezing the pieces together tighter and tighter until they form a new rock - a sedimentary rock.

Finally, metamorphic rocks are rocks that have gone through a big change. Remember when we talked about caterpillars turning into butterflies? That process is called metamorphosis, and metamorphosis just means changing shape, and that's exactly what these metamorphic rocks have done. They change when they're exposed to thing like really high heat or really intense pressure or sometimes both. This usually happens deep within the Earth, where rocks can get squished, bent, folded, but not melted.

So those are the three main kinds of rocks, but how can you tell one from the other? It can be pretty hard, but sometimes we can find clues in the rock about how it formed, from how it looks or feels. So what do you guys think, do you want to play rock detective?

OK. Here's our first mystery rock. What kind do you think it is? You see those bendy stripes? This rock looks like it's been stretched and squeezed. It must have gone through quite a change! And in fact, intense heat and pressure gave this rock its squiggly bands. Because it went through a big change inside the Earth, it must be a... metamorphic rock!

Now, what about this one, rock detectives? Check out those layers, kinda like a big cake. Those are layers of sediment that were put down by rivers and oceans over millions of years. Since you can see it's made up of smooshed up layers of sediment, can you guess what it is? It must be a... sedimentary rock!

OK, just one more. This stuff hardly looks like rock at all. Those big goopy loops of black rock look like they're practically melting, and at one time they were. They were made when lava from a volcano spilled into the ocean and solidified into rock. And since this rock came from lava, or magma, you know it's... (Squeaks squeaks) That's right, igneous rock.

So now you know, there's more to rocks than meets the eye. They all have different colors and textures, each one has a story to tell. A story of how it formed. So the next time you're out for a walk, keep your eyes open for the most interesting rocks and play 'rock detective' on your own. And if you find a rock that you think is especially cool, send us a picture at We'd love to see 'em, and we'll see you next time.