Here's a bit more about it:

* Asthenosphere: This is a partially molten layer within the Earth's upper mantle. It's hot and behaves like a very viscous fluid, allowing the rigid lithosphere (which includes the tectonic plates) to move on top of it.

* Lithosphere: This is the rigid outer layer of the Earth, composed of the crust and the uppermost part of the mantle. It's broken into tectonic plates.

Think of it like a giant jigsaw puzzle with pieces (tectonic plates) floating on a thick, gooey substance (asthenosphere).