Sharks have been around for near 400,000,000 years, before dinosaurs. There are more than 350 types of sharks. Almost all are carnivores, and chow down on sea life (That's right, humans aren't part of a regular diet for sharks) as they slowly cruise in the water.
A shark's skeleton is made of tough and flexible stuff, like your ear. It has anywhere from 5-7 pairs of gills. These absorb oxygen into the blood.
You can be hurt by just making contact with a shark. These spiky things are called denticles. These are shed and replaced, just like teeth.
Sharks have a lifetime supply of teeth. Adults go through 7-12 sets in a year. Different species have the kind of teeth they need to survive. Sharks don't chew with these powerful teeth though, they swallow things whole or in large pieces. Their design is perfect for hunting. They hear, feel, or smell everything in the water from far away.
Some species have 1-100 babies at a time. Some lay eggs, others have live babies. They find safe places, nurseries to give birth. Many pups swim to shore to grow up on their own. They don't fully grow until 10-15 years old, and some live to be 100.
Tiger sharks are the most dangerous sharks. They are common, hunt in shallow water, and eat anything that fits in it's mouth. The Great White has been known to attack boats until they sink. The Whale Shark can grow to 54 feet, while the Spined Pigmy Shark only grows to 6 inches.