The carbon-14 isotope enter the biosphere when carbon dioxide is take up in plant photosynthesis. Plants are eaten by animals, which exhale carbon-14 in CO2. Eventually, carbon-14 participates in many aspects of the carbon cycle. The carbon-14 lost by radioactive decay is constantly replenished by the production of new isotopes in the atmosphere. In this decay-replenishment process, a dynamic equilibrium is established whereby the ratio of carbon-14 to carbon-12 remains constant in living matter. But when an individual plant or an animal dies, the carbon-14 isotope in it is no longer replenished, so the ratio decreases as carbon-14 decays. This same change occurs when carbon atoms are trapped in coal, petroleum, or wood preserved underground, and of course, in Egyptian mummies. After a number of years, thereare proportionately fewer carbon-14 nuclei in, say, a mummy than in a living person