Here's why:
* The mantle is a thick layer of mostly solid rock, located beneath the crust.
* It's hot and under immense pressure, which makes it behave like a very viscous fluid over long periods.
* This "plastic" nature of the mantle allows tectonic plates (which make up the crust) to move on top of it. This movement is responsible for earthquakes, volcanoes, and mountain formation.
Think of it like a thick layer of gooey caramel on top of a hot stove - the crust "floats" on this moving, hot layer.
