Sheldon Carpenter, age 12, of Rome, N.Y., for his question:

HOW IS A FOSSIL FORMED?

A fossil is a record or remains of a plant or animal that lived in the past. It may be a whole animal preserved in ice, a bone hardened by minerals, a footprint, an insect encased in amber or the outline of a leaf.

Most dead plants and animals are eaten by animals or decayed by bacteria or fungi. Only those buried quickly and protected from decay can become fossils.

Hard parts, such as wood, shells, bones and teeth, decay less easily than soft parts, such as skin and muscles. So hard parts are more likely to be preserved.

Plants and animals that live in dry areas leave fewer fossils than those that live in damp places or in water. Dissolved minerals in the water help form fossils.

Fossils may be preserved in several different ways. The chief kinds are petrified fossils, molds and casts, prints and whole animals and plants.

Petrified fossils are the remains of plants and animals that have turned to stone. Water dissolves away the original substance of the plant or animal and as the substance dissolves, minerals replace it. Minerals fill in the small air spaces in bones or shells without changing the original shape of the object.

Living things sometimes become buried in mud, clay or other materials that hardens around them. Later, the bodies dissolve away, leaving openings within the hard material that are natural molds of the original.

Prints may be molds of thin objects, such as leaves or feathers, or they may be tracks or footprints left by extinct animals. Prints are preserved when the soft mud in which they are made hardens into stone. Some prints show veins and pores in leaves.

Whole animals and plants are rarely preserved. Most fossils consist only of shells, teeth, bones and other hard parts. Flesh almost always decays too quickly to be preserved.

Most fossils are found in sedimentary rocks, or rocks that have been built up in layers from small particles. These rocks lie beneath about three fourths of the land surface of the earth. Most of them contain fossils.

To find a certain kind of fossil, a scientist must go where the rocks that contain this kind of fossil lie near the earth's surface.

The best collecting areas are places where wind and water have cut deep into rocks and exposed large areas. Fossils lie nearer to the surface in these places.

Fossils tell about the history of life on earth and of the past changes in geography. Since fossils show a continuous array of living things for the last 600 million years, we know that the climate for that time has been uniform enough to support life.

Fossil shellfish indicate that the rocks that contain them formed under the ocean.'