Humans pose a greater danger to sharks than sharks do to humans. People kill about 100 million sharks each year, eating shark fin soup and shark meat, using shark oil cosmetics, wearing sharkskin boots and shark tooth jewelry, and using parts of the shark to make medicines. Most fish have bony skeletons, but sharks have skeletons made of cartilage. Also, throughout their lives, all sharks grow new teeth, with one set of teeth constantly growing in behind another. And instead of scales, sharks have a rough surface covered with denticles or skin-teeth.
Sharks come in many sizes.
Some sharks lay eggs from which young sharks, called pups, hatch. Other species give birth to live pups. Pups fend for themselves from the first day, growing slowly to adult size over as long as 15 years. Fossil records show that the earliest sharks lived nearly 400 million years ago. Despite their long history on earth, many mysteries about them remain.