Here's why:

* The Asthenosphere: This layer is located in the upper mantle, beneath the lithosphere (which includes the crust). It's characterized by being partially molten, allowing it to behave like a very viscous fluid over long periods. This fluidity is crucial for plate movement.

* Plate Movement: The rigid lithosphere, which includes the crust, floats on top of the asthenosphere. Convection currents within the asthenosphere, driven by heat from the Earth's core, cause the plates to move slowly over time.

Think of it like a giant conveyor belt: the asthenosphere's movement is like the belt, and the lithospheric plates are like boxes riding on top.