Radiocarbon dating relies on a simple natural phenomenon. As the earth's upper is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon-carbon (C-14).

The unstable isotope is brought to earth by atmospheric activity, such as storms, and becomes fixed in the biosphere. Because it reacts identically to C-12 and C-13, C-14 attached to complex organic molecules through photosynthesis in plants and becomes their molecular makeup. Animals eating those plants in turn absorb carbon$-14$ as well as stable isotopes. This process of ingesting $\mathrm{C}-14$ continues as long as the plant or animal remains alive.

The $\mathrm{C}-14$ within an organism is continually decaying into stable carbon isotopes, but organism is absorbing more $\mathrm{C}-14$ during its life, the ratio of $\mathrm{C}-14$ to $\mathrm{C}-12$ remains about same as the ratio in the atmosphere. Where the organism dies, the ratio of C-14 within its carcass begins to gradually decrease.