Carbon 14 is an isotope of carbon that is created naturally in the atmosphere by cosmic rays which in turn is breathed by living things or absorbed by plants. Over a lifetime of breathing, plants and animals maintain a natural ratio of carbon-14 to carbon-12. When the living thing dies, it stops absorbing the carbon-14 from the air.
Carbon-14 decays into nitrogen by beta decay over time, but the process takes a long time. The half life of carbon-14 is 5,720 years, meaning after 5,720 years, the organism will have half the carbon-14 levels it did when it was alive. If you measure the amount of carbon-14 in a once living object, you can determine the approximate age of the organism.
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