The purpose of using the half-life concept is to be able to date things that reach back into the geological time scale. A half-life is the amount of time it takes for a sample of a radioactive element within an organism to have it's nuclei decay to a more stable form by one-half. A good example would be the use of carbon-14, which is unstable, and takes 5000 years for half it's nuclei to deteriorate to a more stable nitrogen-14. Therefore, if a scientist knew the starting amount of carbon-14 in a sample, and measured what was left, the scientist could get a pretty accurate idea of the age of the sample. It should be noted that carbon-14 dating measures decay in an exponential rate.