Here's why:

* The Earth has multiple layers: The Earth's structure is divided into several layers: the crust, the mantle, the outer core, and the inner core.

* The crust is the outermost layer: This is true, but it's not the only solid layer.

* The mantle is mostly solid: While the mantle is mostly solid rock, it behaves like a very viscous fluid over long periods. This is due to immense pressure and heat.

* The inner core is solid: This is because the pressure is so immense that even at extremely high temperatures, the iron and nickel are packed tightly together in a solid state.

Therefore, the crust is not the only solid layer on Earth.