Let's suppose there is a chessboard (with 64 squares), and there is a penny on each square. Each of the pennies has a side (head or tails) facing up, chosen at random.
I look at the board and choose my favorite penny. I then tell my two mathemetician friends that I will reveal which penny is my favorite to one of them, who then must flip over exactly one of the pennies on the chessboard, while the other is in another room not looking. Once the coin is flipped over, the uninformed mathematician is summoned into the room and must deduce which penny is my favorite only by examining the coins on the chessboard.
What strategy can the two mathematicians devise to ensure that my favorite penny can always be deduced?