Here's why:

* Asthenosphere: Located beneath the lithosphere, the asthenosphere is a semi-solid layer of the upper mantle. It behaves like a very viscous fluid over long periods. This plasticity allows tectonic plates to move on top of it.

* Lithosphere: The rigid outer layer of the Earth, including the crust and the uppermost part of the mantle, is called the lithosphere. It's not plastic; it's brittle and breaks under stress.

* Other layers: The lower mantle is solid but can deform under pressure. The outer core is liquid, and the inner core is solid due to immense pressure.

In summary: The asthenosphere is the only layer of the Earth with plasticity, which is crucial for plate tectonics and the movement of continents.