The age of organic material determined by the amounts of carbon isotopes 12, 13 and 14. The ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. Carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. Carbon 14 is an radioactive isotope that decays radioactively until none is left; . Thus, the ratio records the time elapsed since death. Since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years.