Crust
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.
simple:Crust