The plates of the Earth's lithosphere (which includes the crust and the uppermost part of the mantle) slide over a layer called the asthenosphere.

Here's why:

* Asthenosphere: This layer is located in the upper mantle and is characterized by being partially molten, making it more fluid and less rigid than the lithosphere above.

* Lithosphere: The lithosphere is the rigid, outermost layer of the Earth that includes the crust and the upper mantle.

Because the asthenosphere is less rigid, the lithospheric plates can move over it like a giant conveyor belt. This movement is what drives plate tectonics, resulting in earthquakes, volcanic activity, and the formation of mountains and ocean basins.