* Crust: The outermost layer, thin and rocky.

* Mantle: The thickest layer, mostly solid rock but with a partially molten layer.

* Outer Core: Liquid iron and nickel.

* Inner Core: Solid iron and nickel, extremely hot and under immense pressure.

However, some people might argue that there are five layers, including:

* Lithosphere: This includes the crust and the uppermost part of the mantle, which behave as a single, rigid layer.

So, depending on how you classify the layers, the answer is four or five.