Here's why:

* The Earth's structure: The Earth is made up of several layers:

* Crust: The outermost, thin, and rocky layer. It's divided into oceanic and continental crust.

* Mantle: A thick layer of mostly solid rock, but it behaves like a very viscous fluid over long periods.

* Outer Core: A liquid layer of iron and nickel.

* Inner Core: A solid ball of iron and nickel, despite the incredibly high temperatures.

* Solidity vs. Rigidity: While the mantle is mostly solid, it's not rigid like the crust. It can flow slowly over millions of years, allowing tectonic plates to move.

In conclusion: The Earth's crust is the outermost solid layer, but the mantle also contains solid rock, though it behaves like a viscous fluid over geological time.