" This method measures the difference between the amount of potassium in the rock and the amount of argon in it, because potassium decays and becomes argon over time. But it takes a very long time for the decay to happen, and so this method is best used when something is between two hundred thousand and five million years old. In such cases, it would also be impossible to use radiocarbon dating, which works on organic remains but not stone tools and is useful only for dating things within the last fifty thousand years. "

― Eric H. Cline , Three Stones Make a Wall: The Story of Archaeology