Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.
When paleontologist Mary Schweitzer found soft tissue in a Tyrannosaurus rex fossil, her discovery raised an obvious question -- how the tissue could have survived so long?
The bone was 68 million years old, and conventional wisdom about fossilization is that all soft tissue, from blood to brains, decomposes.
Though there is no one age, a good rule is anything over 10,000 years old can be considered a fossil.
If there are too many or too few neutrons, the atom is unstable, and it sheds particles until its nucleus reaches a stable state.How do scientists know the bones are really 68 million years old?Today's knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating.The most widely known form of radiometric dating is carbon-14 dating.This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.