Here's why:

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

* Asthenosphere: A soft, partially molten layer in the upper mantle. It's characterized by its ability to flow over long periods due to the heat and pressure within the Earth.

The asthenosphere's "plastic" nature allows the tectonic plates to move on top of it.