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Crust can refer to the outer layer of a planet, its lithosphere. Planetary crusts are generally composed of a less dense material than that of deeper layers. On partially-molten planets, such as Earth, the crust is floating on fluid interior layers. Such floating crusts are often broken into tectonic plates that, because of fluidity underneath, can move (see continental drift).

Crust can refer to the outer edge of baked bread or the shell of a pie.