Stony corals are the most familiar corals to many people because of the reefs they build. They secrete calcium carbonate to form a cup-shaped, outer skeleton. The living coral (polyp) sticks itself out of the top of the cup to feed. Over time, the polyp secretes more calcium carbonate at the base, raising it higher, and leaving layers of cup-shaped chambers below. While some stony corals live alone (solitary), many live in groups (colonial). Often, their skeletal material cements them together into a mass, with the polyps inside. Their skeletons form reefs made of layers like a condominium with living corals at the top. The reef-building stony corals live mostly in warm, tropical waters. Solitary stony corals, less well-known, are more evenly distributed in the oceans of the world, even in the total darkness of ocean depths. Some are tiny like grains of rice, while others are fist-sized.