Dee Dee Dulong, age 14, of Lansing, Mich., for her question:

HOW ARE SHELLS MADE?

There are about 100,000 different kinds of mollusk shells and all are formed in just about the same way. The shell has three layers: an outer, middle and inner layer. Each layer contains a form of calcium carbonate, a kind of limestone also found in marble and other rocks.

Food eaten by the mollusk provides the minerals that form the shell and give it color. The bloodstream of the animal carries the minerals to the mantle, a fleshy skinlike tissue inside the shell. Special glands in the mantle produce the substances in the form of a liquid that make the shell. Other glands add a hardening material so the liquid quickly becomes firm and strong. Still other glands produce the color.

As long as the animal grows, its shell also grows. Clams and snails begin to grow shells before they hatch.