This video explains the carbon-14 method for determining the age of an object containing organic material by using the properties of this radioactive isotope of carbon.

Carbon has two stable non-radioactive isotopes, carbon-12 and carbon-13, and one radioactive isotope, carbon-14. The half-life of carbon-14 is 5730 years. Because of its relatively short half-life you might expect that its concentration in the atmosphere would be reduced over thousands of years but new carbon-14 is always being produced in the upper atmosphere by the interaction of cosmic rays, neutrons and nitrogen-14 atoms. Therefore the concentration of carbon-14 in the atmosphere is relatively constant.

Plants and animals have the same concentration of carbon-14 as the atmosphere because they are constantly exchanging it with their environment. When they die, the carbon-14 in their bodies continues to decay, but they are no longer able to take up new carbon-14 and the concentration of carbon-14 in their bodies decreases. Knowing the concentration of carbon-14 at the time of death and the concentration of carbon-14 some time (t) after death enables you to determine how long it has been since the plant or animal died.

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