A small amount of carbon in our atmosphere and the bodies old plants and animals is radioactive Carbon-14.
Measurements from radioactive carbon can be used to date old, once-living materials, such as wood.
The activity of radioactive carbon can be used to find the approximate age of a once-living material. The amount of radioactive Caron-14 in the atmosphere has remained unchanged for thousands of years. A dead object doesn't exchange gases with the air as living matter does. As the Carbon-14 in the dead object decays, it is not replaced so the radioactivity of the sample decreases.
So, the dead object will have a different radioactivity to living matter. The ratio of these two activities can be used to find a fairly accurate approximate age for the objects within known limits (approximately 50 years).
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