 We can use what we know about uranium decay to date rocks containing uranium. One significant complication arises when dealing with rocks compared to a controlled laboratory environment. In the lab, we controlled the amount of atoms in a sample. But to accurately measure the number of atoms in a sample pulled from a rock taken from the crust of the earth can be quite difficult. But there is a straightforward way to determine the number of atoms in a sample like this. All we need to do is add a measured amount of a different element, take a sample of the mixture and measure the ratio of the sample to the additive. This process is called isotope dilution. I have used carbon to illustrate the concept. In actual measurements isotopes of uranium or lead not found in nature are used.