Eddie Claus, age 11, of Pittsburgh, Pa., for his question:

CAN CARBON BE USED TO TELL THE AGE OF A FOSSIL?

The element carbon occurs in several different forms called isotopes. One of these isotopes can be used to reveal the age of an old bone or an ancient log. It works because it happens to be radioactive  and all radioactive substances decay at a steady, unchangeable rate.

All living cells contain carbon, and the carbon remains when long dead plants and animals become fossilized. Most of this material is ordinary carbon, which has an atomic weight of, 12.

However, in every sample of ordinary carbon there are a few isotope atoms. One of the heavier isotopes is carbon 14, which loses its extra weight by radioactive decay. There is one of these radiocarbon atoms among every trillion atoms of ordinary carbon atoms.

While they live, plants and animals consume a one in a trillion ratio of radiocarbon, along with ordinary carbon. When they die, the intake stops, but the radiocarbon continues to decay at a precise rate. During the first 5,680 years, exactly half of the original material disappears. During the next 5,680 years, half of the remaining quota disappears, and so on.

The half life of radiocarbon is 5,680 years; during which time half of any sample decays. Naturally, this changes the one in a trillion ratio of radiocarbon and ordinary carbon. This gives the key to the carbon dating of fossils.

For example, suppose archeologists find fossil bones and logs in an ancient campsite. They test to find the ratio of ordinary carbon to radiocarbon. Let's suppose that there is one radiocarbon atom per two trillion ordinary carbon atoms. This would mean that half the original radiocarbon has decayed, which must have taken 5,680 years. Hence, the campsite fossils date back 5,680 years.

Naturally, most samples involve complicated fractions of the half life period. What's more, after 60,000 years, only a 1,000th part of the original radiocarbon remains. This tiny quota is too hard to detect, which is why carbon cannot help to date fossils older than half a million years or so.