Vincent P. George, age 15, of San Diego, California, for his question:

How does carbon dating work?

If you spend exactly one dollar each day, when you are 365 dollars poorer you are one year older. Obviously there are simpler ways to figure your age. But the months cannot tell the age of a log and the calendar cannot date an old bone. For these tasks we need a modified form of the dollar a day method. In one method, we substitute 5,750 years for a day and the steadily spendable dollars are isotope atoms of carbon 14.

Carbon 14 is called radiocarbon because its isotope atoms are radioactive. Ordinary carbon has an atomic weight of about 12. Carbon 14 is like ordinary carbon except it's heavier and loses its extra weight by radioactivity. Because it breaks down at a steady, dependable rate, it can be used to date logs and old bones.

Ordinary carbon is present in every living cell. Some of its atoms are heavier isotopes of carbon 14. Scientists assume that in all forms of carbon, one atom in a trillion happens to be one of radioactive carbon 14.

These isotopes are everywhere. Plants and animals consume them with air and food, along with ordinary carbon. Meantime, the consumed radiocarbon breaks down at a steady rate while more is added from cosmic rays smashing portions of nitrogen atoms. This balanced gain and loss continues until the life processes stop. Then no more is added, though the radiocarbon quota in the tissues continues to break down and disappear.

All radioactive decay proceeds in predictable stages at a steady predictable rate. During 5,680 years, precisely half of the atoms in a sample of radiocarbon break down and disappear. This period is the half life of carbon 14. After 11,360 years, only one quarter of the original sample is left. After 60,000 years, only one thousandth part of the original quota remains.

Suppose archeologists find bones and building timbers in a prehistoric campsite. They test to find the carbon 14 content still present in the once living tissues. Let's suppose that tests reveal one radiocarbon atom per two trillion ordinary carbons. Half the original radiocarbon has had time to decay which means that the bones and trees stopped living about 5,680 years ago.

After 60,000 years, there is one part radiocarbon per 1,000 trillion parts of ordinary carbon. Such a tiny quota is too hard to detect precisely which is why carbon 14 cannot be used to date the remote past.

Nor can it be used to date minerals or other inorganic substances. It is limited to fossil remains of once living plants and animals. And the tiny traces of decreasing radiocarbon make it difficult to date fossils much older than a few thousand years. What's more, some scientists complain that variations in cosmic rays and other factors tend to upset the carbon 14 system of dating. At best, it gives only the approximate age of this old log or that old bone.