A guy goes into a doctor office and says, "I think I have a tapeworm, doctor. I keep eating and eating but I don't gain any weight!" The doctor says, "Come see me tomorrow, and bring a banana, and a cookie."
So the man buys a banana and a cookie on the way home, and goes to the doctor the next day. The doctor says to him, "Okay, give me the banana and the cookie, and pull down your pants." "Are you sure?" "Positive." Grudgingly, he pulls down his pants. The doctor shoves the banana right up his ass. He waits a few minutes, and up goes the cookie.
The doctor says, "Okay, now tomorrow, I want you to do the same thing." So he does. And this goes on for about a week.
On the second to last day, after treatment, the doctor says, "For tomorrow, I want you to bring a banana and a hammer." Fearing the worst, but wanting to get rid of the tapeworm, the man does so.
The next day, up goes the banana. The doctor waits a few minutes, and then out pops a sign that says, "Hey! where's my cookie?!" And well, I think you can figure out what the doctor did with the hammer.