Cosmic radiation generates neutrons in the atmosphere, which split nitrogen 14 into hydrogen and carbon 14. Plants consume carbon dioxide, including small quantities of radioactive C14 atoms. When those plants, the animals that ate them, and the animals' predators die, they stop consuming the C14 isotope, which decays to half its original amount in about 5730 years. Observing the number of decay events per minute in fresh and historic samples makes it possible to estimate the age of a sample.