On the first side of a piece of paper write the words “The statement on the other side is true.” Now turn the paper over and write “The statement on the other side is false.” Let us consider first Side 1 asserting that Side 2 is true. If Side 2 is true, however, then Side 1 is false. But if Side 1 is false, then Side 2 is not true, in which case Side 1 is true.
