Sharks are a group of fish that do not have a bony skeleton, but one made out of cartilage (the sort of thing your nose is made from). Sharks are among the oldest fish, with fossil remains from over 400 million years ago. There are now over 500 species, many of which are threatened by human activities. 100 million sharks are killed by people every year, either from commercial or sport fishing - a number you might find staggering. But sharks are a common seafood in many places, including Japan and Australia. In Australia, fish and chips as a meal often does not mean cod and chips, but shark and chips.

We normally think of sharks as huge, but the smallest shark is tiny - just 17cm long (Dwarf Lantern Shark). Of course, other sharks are very, very large. The biggest is 12m long (Whale Shark). The largest sharks are the world's largest fish.

Sharks have managed to live in all parts of the ocean, from shallow to very deep, from the tropics to the cold polar waters.

The most famous sharks are the Great White Shark, Tiger Shark, Blue Shark and the Hammerhead Shark.

Sharks do not have a swim bladder, which other fish use to keep them still in the water. If a shark stops swimming, it sinks to the bottom. They also need water to flow through their fixed gills slits, which is another reason for staying on the move.

Sharks have teeth that grow from the gums, and they replace their teeth many times during their lives, some as often as every few days or weeks.

Sharks have a very keen sense of smell, and can detect as little as one part per million of blood in seawater. So if a fish or mammal is injured, this can attract sharks from a wide area.

In the video below you will notice that sharks are found in among many other fish. You will find this in all kinds of situations, for example, when antelope are found near to lions. Sharks are at the top of the food chain, and they have no natural enemies, but there is a limit to how much they can eat, so they do not spend all of their time hunting, and other fish know this from the way the sharks behave.

Popular stories tell of solitary killer sharks stalking the oceans in search of people. Of course, this is just for horror books and films. It is not at all the case. A few sharks live solitary lives, but most live in groups and are very sociable. These schools of sharks may migrate thousands of kilometres a year from one hunting ground to another.

Sharks can feel threatened, and this is when they might attack. Threats can include people splashing around or wearing shiny jewellery. But overall only 4 people are killed by sharks world-wide each year (less than are killed on the world's roads each minute).

Some sharks are incapable of attacking other fish, let alone people. Basking Sharks, Whale Sharks, and Megamouth Sharks are filter feeders and can eat only plankton. Many sharks that live close to the sea bed live on shellfish, lobsters and crabs which they crush with their teeth. Most other sharks swallow their prey whole unless it is too big. Then they bite chunks out of it.

Sharks swim at about 8km/hr, faster than most small boats.