a quarter

I was listening to these two kids in class, boys. They were around twelve or so, and one was telling the other this joke that went, "A man rubs this bottle and a genie comes out and says - you have three wishes, but for every wish your wife gets double. So the man wishes for a nice car, and his wife gets two. Then he wishes for a nice house, and his wife gets two. Then the man says - I wish to be beaten half to death..." I watched as the kid who was telling the joke waited expectantly, and nothing came. The other kid just sat there blank-faced. While I was mulling over the various undertones of the joke the joke-teller said, "Don't you get it? What's double of half?", and the blank-faced one looked back at him, still no expression on him, and said, "A quarter?"