Radiocarbon dating — more formally, carbon-14 dating — works because all living organisms incorporate a specific radioactive isotope of carbon, carbon-14, from the atmosphere during their lifetimes.

Carbon-14 is produced when cosmic rays strike nitrogen atoms in the upper atmosphere, and it circulates through the carbon cycle — from the atmosphere to plants through photosynthesis, from plants to animals through eating, and so through the entire food web.

As long as an organism is alive, the ratio of carbon-14 to stable carbon-12 in its tissues reflects the atmospheric ratio of those isotopes.

When the organism dies, it stops incorporating new carbon.