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With rock samples, the scientists first analyze the chemical composition of a rock. If enough of a particular isotope exists in the rock, it will be used for the isotopic dating. If possible, they will use several different isotopes and compare the results. An important thing to remember is the half-life of a radioactive isotope means that after that amount of time, approximately half of the isotope will have decayed into something else. Using K-40 as the example again, it has a half-life of 1.25 billion years and decays into Ar-40. So, after 1.25 billion years approximately 1/2 of the K-40 will have decayed into Ar-40. After another 1.25 billion years, half of the remaining K-40 will have decayed (so that you only have 1/4 of the original K-40). So what they really do is not just measure the amount of K-40 in the sample, but also of Ar-40. Using this, they can determine how much K-40 was in the rock when it was formed. The ratio of current K-40 to original K-40 gives them the time that has passed since the rock was formed.

Using this, they can determine how much K-40 was in the rock when it was formed.

Thanks for answering.

OK, I follow. Actually, I pretty much understood this already. The part I am stumped on is the difference between rock and living matter. With living matter we can know pretty well when it died because we are measuring C14 from the carbon it ate - when it died, it stopped eating.

Rocks don't eat. They do get formed, but they get formed out of something else. If I followed the story correctly, they were claiming that all the rocks landed on the moon at about the same time because they were all aged the same. How does that get proven with K40 (or whatever) dating?

It seems to me that all we know is that the original matter that the rocks came from was formed based on the age. But does that prove those rocks were formed then, or just the planet? Wouldn't whatever the rock comes from have already started decaying? Would a newly formed rock have the already aged mix?

That's my problem with understanding. Rocks don't eat. They are just combinations of earlier things.

As for aging the craters based on the age of the rocks, that seems beyond foolishness to me. Who knows how long some of those rocks were floating around before they intersected.

Anyway, thanks for the answers. Hopefully there is one more left.