Coral reefs are made up of millions of coral polyps stuck together. A coral polyp is the shape of a tube and has a stomach, a mouth, and tentacles around the mouth. A polyp  takes calcium out of the water, and with that calcium it makes a shell around itself. This shell is the coral reef's skeleton. Coral polyps need sunlight and oxygen to survive. Coral polyps also need moving water to carry plankton to them so they can eat it. Inside the polyp stomach is algae which eats the waste products from the polyp. The coral gets its color from the algae (algae is different colors, that is why coral are). When a reef dies, it turns white. The algae also contains chlorophyll which makes oxygen from the carbon dioxide in the water.