Little Billy is sitting on the toilet reading a comic book. But about every 10 seconds or so, he puts the book down, grips onto the toilet seat with his left hand and hits himself on top of the head with his right hand. His mother notices him and says, "Billy, are you all right? You've been in here for a while." Little Billy says, "I'm fine, mummy. I just haven't done it yet." His mother says, "OK, you can stay here a few more minutes. But, why are you hitting yourself on the head?" Little Billy replies, "It works on the ketchup bottle!"