Andy Tolzman, age 14, of Burlington, Vt., for his question:

HOW DOES A FISH GILL WORK?

A fish gill is the breathing organ of many animals that live in the water. It does the same work as the lung of an animal that lives on land. Most fishes breathe with gills.

when a fish breathes, it opens its mouth at regular times and draws in a mouthful of water. It then draws the sides of its throat together. This forces the water through the gill openings. The water passes over the gills to the outside. Valves in the mouth keep water from escaping through the mouth again.

The gill itself has the form of plates, branches or threads of very tiny membrane. They are so thin that oxygen from the water can pass through into the blood vessels of the gills.

The oxygen is then carried by the blood to other parts of the body. Carbon dioxide, a waste product, passes from the blood through the gills into the water.

Gills or gill like organs are found also in mollusks, crustaceans and some insects.