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, there are proportionately fewer carbon-14 nuclei in, say, a mummy than in a living person