Thanks for the answer. That leads to another question. C14 works because something alive "eats" the carbon, and when it dies, no more carbon is collected in it. By measuring the C14, and knowing the half-life, a good guess to the age the thing died can be determined.
What is the process that causes rocks to collect whatever is being used (potassium?), then stop collecting it so the age can be determined? Wouldn't all rocks show the age they were formed? Or could they be formed out of older rocks, and show that age? And if a planet exploded and showered rocks on the moon, would the age be when they were formed or when they showered? Couldn't they shower down at different times, yet show the same age?
I'm sure it is something simple that I am missing, here, isn't it?