Most people are afraid of sharks, but they usually do not know very much about them. For example, there are 350types of sharks, and all of them are meat esters. Some sharks are very big. The whale shark is 50 to 60 foot long. But some sharks are very small. The dwarf(侏儒)shark is only 6 inches(英寸)long. Sharks are 100 million years old. In fact, they lived at the same time as dinosaurs. Today, sharks live in every ocean on the earth, but most sharks live in warm water. They keep the oceans clean because they eat sick fish and animals. Most sharks have four to six rows of teeth. When a shark's tooth falls out, another tooth moves in from behind. Sharks do not have ears. However, they "hear" sounds and movements in the water. Any sound or movement makes the water vibrate. This helps sharks find food. Sharks use their large eyes to find food, too. Most sharks see best in low light. They often hunt for food at dawn (黄昏), in the evening, or in the middle of the night. Scientists want to learn more about sharks for several reasons. For example, cancer (癌症) is common in many animals, including people. However, it is rare (罕见的) in sharks. Scientists want to find out why sharks almost never get cancer. Maybe this information can help people prevent cancer, too.