Here's a breakdown:

* Lithosphere: The rigid, outermost layer of Earth that includes the crust and the uppermost part of the mantle. It's broken into tectonic plates.

* Asthenosphere: A semi-solid, partially molten layer of the upper mantle. It's less rigid than the lithosphere and behaves more like a viscous fluid over long periods. The movement of the asthenosphere is what drives tectonic plate movement.

Think of it like this: Imagine the Earth's surface is a hard shell (the lithosphere) floating on a partially melted layer (the asthenosphere). The movement of the molten layer causes the shell to shift and break, leading to earthquakes and volcanic activity.