Here's why:

* Lithosphere: The lithosphere is the rigid, outermost layer of the Earth. It includes the crust (the thin, rocky outer layer) and the uppermost part of the mantle. Think of it like the Earth's strong, solid shell.

* Tectonic Plates: Tectonic plates are large pieces of the lithosphere that move and interact with each other. These interactions are responsible for earthquakes, volcanoes, mountain ranges, and many other geological features.

So, the tectonic plates themselves are essentially pieces of the lithosphere. They "float" on the asthenosphere, which is a semi-molten, deformable layer in the upper mantle. The movement of the asthenosphere due to convection currents (heat transfer) is what drives the movement of the plates.

In summary: Tectonic plates are made of lithosphere, and they move on top of the asthenosphere.