The method uses the radioactive isotope carbon-14. Most organic matter contains carbon. Carbon has different isotopes, which are usually not radioactive. C is the radioactive one, its half-life (time it takes to reduce its radioactivity by half) is about 5,730 years. This makes it possible to tell the age of substances that contain carbon. The method works to about 60,000 years old. Dates obtained are usually written as "before present" ('present' is 1950). 