Inside their skin, sharks are different from most fish, too, because their skeleton is made up of cartilage, which is softer and more flexible than bone. Some sharks have five rows of teeth. When one tooth breaks, another tooth which was lying down right behind the broken one, pops up, ready to work. If our sense of smell were as good as a shark's, we could tell what our friends across town were having for dinner. Sharks can smell some substances that are as diluted as one part odor to one billion parts of sea water.
Sharks are also very, very sensitive to electrical fields.
Every living body gives off a small electrical field around its body. Fish-eating sharks have an important job. In a roundabout way, they keep fish healthy. Sharks eat sick, injured or weak fish because they're easier to catch. This way, other fish don't get sick and weak fish don't reproduce, making more weak fish.