In the early 1900's the concept of radioactivity surfaced and offered a new and much more accurate way to date rocks. Radiometric dating is a method commonly used in modern times to help date rocks and fossils. New rocks and fossils contain a certain amount of radioactive isotopes. As these rocks age, the isotopes lose energy at a regular and predictable rate. When half of the once-radioactive isotopes have become stable (not radioactive), this is referred to as the half life. Scientists know what many common elements' half-lives are. They are then able to count the percent of unstable (parent) isotopes compared to the stable (daughter) isotopes and have a very accurate date as to when the rock or fossil formed or was deposited.