Here's why:
* The asthenosphere is partially molten: While not completely liquid like magma, it's hot enough to behave like a very viscous fluid over long periods. This allows the rigid tectonic plates to move and interact.
* Density difference: The lithosphere (which includes the crust and the uppermost part of the mantle) is denser than the asthenosphere. This density difference allows the lithosphere to "float" on top of the asthenosphere.
* Convection currents: The heat from Earth's core creates convection currents in the asthenosphere, which in turn drive the movement of the tectonic plates.
Think of it like a giant, slow-moving conveyor belt!
