Carbon dating is a technique used by scientists to determine the approximate age of once living materials. It is based on the decay rate of the radioactive isotope of carbon, which is carbon-14. Carbon-14 is a form of carbon taken in by all living things while they are alive. Carbon-14 is produced in small quantities in Earth's atmosphere, as a result of interaction between the atmosphere and cosmic rays from space. All living things absorb carbon-14 while they are alive; they cease to absorb the element when they die. Carbon-14 has a "half-life" of 5730 years, meaning it takes that long for half the nuclei present to degrade to a more stable, less radioactive nitrogen-14. Scientists compare the amount of radioactive nuclei present in the organic sample to the amount of carbon-14 available in the atmosphere. Once that calculation is made, the number of half-lives that has passed may be calculated. After that, the number of half-lives times 5730 years gives the approximate age of the organism. Carbon-14 dating was developed by an American chemist, Willard Libby, 1908-1980.