 Determining the age of rocks is called geochronology, but before the early 20th century there was no way to do it. In the 1920s, a British geologist named Arthur Holmes, the father of modern geochronology, came up with an accurate method called radiometric dating. The method requires that the rock contain at least some measurable amount of radioactive materials such as uranium and the lead it decays into. To understand how we can use the uranium to date rock, we'll go one level deeper into just how this dating method works. We'll cover how we know what uranium radiates, how we know how long it takes uranium to decay, and how we measure how much uranium has decayed into lead in a rock sample. We'll start with radiation itself.