A shell is a hard casing made by an animal. A wide variety of animals use shells, and shells come in many shapes and sizes. Most shells consist of a spiral, or a plate-shape. They show ridges where the shell has been enlarged each year to make room for the animal as it grows.

When an animal with a shell dies, the soft parts rot away, but the shell remains for a long time because it is basically made of a kind of limestone. Shells also have to be able to stand up to quite strong pressures, such as you find when waves break on a shore, or rivers are in flood. For this reason, shells remain for a long time after the animal has died, and they are the most common fossils in rocks.

The shell is actually a skeleton. We have skeletons inside our bodies, but most of the world's creatures are not like this and have skeletons on the outside. All of their soft parts are fastened to the shell.

Animals with shells include snails, limpets, winkles, mussels, crabs, lobsters, clams and so on.