Perhaps one of the greatest staples of grade school humor, the "What are you eating under there?" joke has been passed amongst children for a long time, and presumably still is.
The joke goes like this: Bobby walks up to Billy on the playground and asks: "What are you eating under there?" Billy, of course, is not under anything, so asks quizzically: "Under where?". Which of course, sounds like "underwear". Then Bobby laughs at Billy and says "you are eating underwear?".
This joke is amusing, up to (so far) my current age of 22 because it requires a deft touch of timing and surprise, and the results are squared since the punchline is provided not in a static monologue but in a dynamic interpersonal fashion.