A shell is the hard outer covering of a living thing. It provides protection to the organism. It is either made by the creature itself or by its parent. Shells are composed of calcium carbonate (lime) and silica or lignin (woody material). Most common shells are eggshells, shells around the body of a crab or lobster, or shell over the body of a turtle. The shells of crabs and lobsters are made up of plates joined together, to allow movement. Turtles’ shells are part of their bodies which are made of bones covered with scales.