Fossils can even be dated. We can tell how old they are, mostly by measuring radioactive isotopes in the rocks. We’ll learn what isotopes are, and atoms, in Chapter 4. Briefly, a radioactive isotope is a kind of atom which decays into a different kind of atom: for example, one called uranium-238 turns into one called lead-206. Because we know how long this takes to happen, we can think of the isotope as a radioactive clock.
