An oyster belongs to the mollusk family. They live in shallow water by reefs or sand bars. In this habitat they grow from a baby, or what the scientists call a spat, the size of a pencil dot. At 2 1/2 years of age the oyster is full-grown at 3 inches long. Some oysters form pearls from grains of sand that get inside the shell. The oyster covers the sand with a smooth, shiny  substance to keep it from irritating, like a stone in your shoe.