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Oyster shells are made of calcium carbonate (lime). The oysters get the chemicals to make their shells from the water they live in. They also have a sort of skin, called a mantle, which spreads this calcium carbonate on the outside of their bodies to form a protective shell. Oysters must live in water that is temperate (warm all year) and not too cloudy. They grow only in areas where salt and fresh water mix together, like salt marshes. Oysters are born as free-swimming plankton (tiny microscopic organisms). When they grow up, they find a place (on mud, coral, trash, or other oyster shells) to attach and grow. Once they grow their shells, they can’t move around anymore. When the tide is high, oysters are covered by water, but when the tide goes out, they are left sticking up into the dry air. Their shells close tightly together so the animal inside will not dehydrate (dry out) before the tide comes back in.