After you have dealt about half of the 52 cards for a bridge game (with four players), the phone rings. You put down the undealt cards to answer the phone. When you return, none of the cards has been touched, and no one can remember where the last card was dealt. Without counting the cards in any hand, or the number of cards yet to be dealt, how can you finish dealing rapidly and accurately, giving each player exactly the same cards they would have received if you hadn't been interrupted?
Deal from the bottom of the deck, starting with yourself and deal backwards (which should be counterclockwise). Since the only thing you really know is that you would have received the last card, this is what you need for a starting point.
Today's brain teaser courtesy of Braingle.com.