Carbon 14 is formed in atmosferic air when neutrons from cosmic rays collide with nitrogen nuclei in this equation (14/7)N + (1/0)n -------> (14/6)C + (1/1)p This radioactive Carbon formed react whit Oxigen of air forming radioactive Carbon dioxide. This radioactice CO2 is absorbed by vegetals (fotosinteses) and by animals (food). Thus the amount of C14 in animals and vegetals is almost constant because at the same time it is absorbed it too vanishe by Beta decay. (14/6)C -------> (14/7)N + (0/-1) Beta particle Whem the organisme dies, the C14 is not replaced by food or fotosinteses and the amount of this element start decreasing. Knowing the Half-life period of C14 (5.730 years) the scientists can determine the age of a fossil by the relation C14 remaining to the amount existing in a current similar species.