Did you hear about the constipated mathematician? He worked it out with a pencil.
@MorganFreeman has just reminded me of the bloke in the who says to the barmaid "there's no paper in the toilet". So she takes a sheet of newspaper, folds it into four quarters, rips the central corner off, unfolds it to reveal a hole in the centre of the paper, hands both pieces of paper to the customer and starts to turn away. "Whoa, what's this?" Says the customer. "You men are dumb" she says "you put your middle finger through the hole and wipe your bum". "Well what's this little piece out of the centre for?" "Simple, that's for cleaning your finger nail!"