Bob posted this puzzle many years ago (and I remember it from logic class):
Suppose the letters and numbers below are on separate cards. The cards have a number on one side and a letter on the other, and someone tells you: “If a card has a vowel on one side, then it has an even number on the other side.”
Which of the cards would you need to turn over in order to decide whether the person is lying?
E K 4 7