Answer:
Different elements have different half lives, which means that certain elements are only useful for determining the age of a rock up to a certain age. Carbon-14, for example, is great for dating archeological finds as old as 50,000 years. But that's the limit of its usefulness. I would not be useful for dating dinosaur bones. For that you would need an element with a much longer half life, such as Uranium-238, which has a half life of more than a million years.