Any alive being consumes carbon. As long as we are alive, the carbon-14 of which our cells are made up disintegrates out of carbon 12, but we consume in same time the same quantity of carbon-14. so we always have the same proportions of carbon 12 and 14. Of more, this invariant ratio during our life is also invariant of an individual with another, and one time with another (almost). One can thus know the value of the ratio at the time of died of a plant or an animal.
On the other hand, when one dies, carbon-14 continues to disintegrate whereas we do not absorb any any more. The ratio is thus modified, according to a relatively simple function of tilme. By measuring this ratio for a fossil or a mummy, one can thus easily calculate his age.

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