When I was in 4th grade, the teacher had us read Stone Soup, and then we made stone soup.

The story goes like this: A poor beggar bets this man that he can make a delicious soup using only a stone. Then the beggar says something like, “You know, this soup would taste just a bit better with some potatoes.” so the man rushes off to get potatoes to put into the boiling water with the stone. And the beggar keeps asking for one more vegetable, and another, and another, until finally the guy is making vegetable soup with a stone in it.

My point, and I do have one, is that the stone isn’t the important bit. It’s the things that are added on to it after which are important, even though the hapless man is conned into thinking the stone is the important bit, the add-ons are just to make it work a little bit better.

Which is exactly how the tortoise cons Achilles. The stone is (A) (B) and (Z).