Radioactive decay
Plants take up 14C by fixing atmospheric carbon through photosynthesis. Animals then take 14C into their bodies when they consume plants (or consume other animals that consume plants). Thus, living plants and animals have the same ratio of 14C to 12C as the atmospheric CO2. Once organisms die they stop exchanging carbon with the atmosphere, and thus no longer take up new 14C. Radioactive decay then gradually depletes the 14C in the organism. This effect is the basis of radiocarbon dating.