Assume you have 27 cards numbered 1-27. You and your partner have do devise some strategy so that for any 4 cards that you pick (without your partner seeing) you'll be able to tell him what they are by showing him only three of them. The only way that you can use the three cards that you show him (other than their respective numbers) is the order in which you present them to him - no patterns or physical properties of the cards are allowed. For example, if there were only nine cards, then for any three that you show your partner the forth card would be 1 out of 6 possibilities (nine minus the three you showed him), since there're 6 ways to order 3 cards you could use the order to tell your partner what the forth card is. The trick is to do it with 27 cards.