Final answer:
Paleontologists determine the age of fossils using relative dating, such as stratigraphy, and absolute dating methods like radiometric dating, which uses radioactive isotope decay rates.
Step-by-step explanation:
Paleontologists use two main techniques to determine the age of fossils: relative dating and radiometric dating. Relative dating is a method which determines the sequence of events in Earth's history without knowing their numerical age. This is often done using stratigraphy, observing that soil is deposited in successive layers or strata, with the oldest at the bottom. Artifact or fossil layers can help indicate the relative age of fossils.
On the other hand, radiometric dating is an absolute dating technique that uses the decay rate of radioactive isotopes to give an approximate numerical age to a rock, mineral, or fossil. This method measures the amount of radioactive material that remains in a fossil and compares it to a known decay rate.