David Newell, age 12, of Ottawa, Ont., Canada, for his question:

WHAT EXACTLY IS A WHITE SHARK?

Lately we have been scared silly by ghastly rumors about the great white shark. He does not attack humans very often, but the famished fellow is known to devour anything from plankton to people. So when warning signs report that he is lurking offshore, no sensible person goes in for a dip. It's nice to know he is merely a fish and cannot possibly grab you if you stay on the dry land.

Several characteristics separate the shark clan from other sea dwellers. One of the major differences between them and the other fishes is that sharks have no true bones; their skeletons are made of a gristly, flexible substance called cartilage. The scientific name for the cartilage fishes is chondrichthyes. Sharks share this classification along with their distant cousins, the skates and the rays.

Almost 250 species of shark have been identified. The great white, up to 20 feet in length and 3 1/2 tons in weight, is not the largest shark. The honor goes to a gentle giant known as the whale shark, who may be up to 60 feet long. However, what the great white lacks in size he makes up in ferocity.

The shark has other features that set him apart from other animals. Perhaps his most outstanding feature is his teeth, which are wedge shaped and serrated like steak knives. They are not firmly embedded in his jaws and, since they have no firm roots, he is forever losing whole sections of his dental equipment.

But this is no problem. A shark has many rows of teeth, and new ones are always in the process of growing. When old teeth fall out or get stuck in a jawful of food, new ones move up from behind to replace them. The teeth are splendid for biting off chunks of live meat, but they are no good for chewing, so each mouthful is swallowed in a single gulp.

The great white shark has tremendous jaws and is capable of swallowing creatures almost half his size. He hunts a wide variety of marine animals such as other sharks, seals, sea lions and sea turtles. Though he rarely preys upon people, he has earned the reputation of being a man eater. He is generally considered to be the most dangerous of all sharks.

Nobody can predict how the great white shark will behave. He may or may not grab at a human being, a boat, an air mattress or anything else in the water. Wherever we find him, in clear or murky, deep or shallow seas, he is a remarkable monster to observe  from a safe distance.