A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
Their method, a type of radiometric dating called uranium-lead (U-Pb) dating, relies on the fact that uranium isotopes radioactively decay to form lead isotopes.
By comparing the amount of each isotope in a sample, the age of the sample can be calculated.
Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.
When I first became interested in the creation-evolution debate, in late 1994, I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.