A shark is a type of fish that has about 350 different species in the world. Sharks are made up of cartilage and are always growing new teeth. Sometimes the teeth grow in rows of 5. Sharks are known to be dangerous creatures, but they do not kill as many people as one would think. Some different sharks are whale sharks, great white sharks, hammerhead sharks, and swell sharks. People are always studying sharks to find out more about them.