The crust is the hard outer layer of something, most commonly bread or the Earth itself.

When you bake bread, the outside becomes firm and golden while the inside stays soft. That's the crust, and some people love its crispy texture while others prefer to tear it off. Pizza has a crust around its edges. Pie has a crust on top, bottom, or both. The crust is what holds everything together and gives it structure.

The Earth also has a crust: the solid rocky outer layer we live on. Beneath our feet lies miles of rock, forming the planet's crust, which floats on the much hotter, partially melted rock of the mantle below. The Earth's crust is thinnest under the oceans (about 5 miles thick) and thickest under mountain ranges (up to 50 miles thick). When pieces of the Earth's crust push together or pull apart, we get earthquakes and volcanoes.

Less commonly, crust can describe any hard layer that forms on the surface of something: the crust of ice on a winter pond, the crust of dried mud on your boots, or even the crust that forms over a healing cut.

A crusty person is someone who seems grumpy or harsh on the outside but might be kinder underneath, like a character in a story who seems mean at first but turns out to have a good heart.