One of my favorite riddles dates back to the 18th century... but it is very much a "turtle riddle", designed to have an obvious wrong answer which, being slightly rude, distracts strongly from the correct answer.
The question is, will folks take offense ("think of the children!") or will they consider it "harmless at worst"?
I'll post the riddle in question here in spoiler markup; if you might be offended by something that appears to reference anatomy, please feel free to skip it and just say "don't."
"A strange thing hangs under a man's clothes. It is long and firm, pierced in front, and has a good fixed place. When a man lifts his garment, it is because he wishes to visit with the head of this dangling instrument the familiar hole which it has, when of equal length, has often filled before."