Explain why scientists believe the Earth developed in a series of layers.

The earth has different layers for a reason similar to chocolate milk. When you put chocolate syrup in milk, where does it immediately go? To the bottom. We don't exactly know how the earth itself formed, but the way it developed its layers is by the heaviest elements sinking to the bottom. The earth's core is made of iron and nickel, with maybe a little sulfur - that's the heaviest part. The mantle is made of heavier minerals in the silicate family, and within the mantle there are certain regions where there is a "phase transition" from one type of crystal structure to another, where the weight of the overlying layers is heavy enough to squish the atoms closer together. The lightest material floats like a crust on top - we call it the crust of the earth, even.

There are different kind of layers in the earth. Really hot ones and ones that cooled down. Google rock layers in the earth. It's super cool to see. These layers could never have formed at the same time under the same conditions