A student is given five random cards from a standard deck of 52 cards. He must choose four cards to reveal to his professor. He does this as follows; the professor is out of the room, and he puts the four cards in four holders on a desk; he has no choice in how he orients the cards, and the order he places doesn’t matter as the professor is out of the room. His goal is to reveal four cards in such a way that the professor knows what the fifth hidden card is. How does he do this?