Tammy Chambers, age 9, of Sparks, Nev. for her question:

WHAT IS THE WORLD'S OLDEST FOSSIL?

A fossil is the record or remains of a plant or animal that lived in the past. The oldest fossils known to scientists are bacteria and algae, one celled plants, that were alive over 3 billion years ago.

The next oldest fossils are many celled plants and animals including those with hard skeletons and shells that left good records of their remains in rocks about 600 million years ago.

Scientists determine when fossils were formed by finding out the age of the rocks in which they lie. They also measure the amount of certain chemicals in the fossils to tell how long the fossils have been in the earth.

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