I present to you a deck of 4 cards. Each card has on one side a letter of the alphabet, and on the other side a single digit from 0-9.
I propose a hypothesis that may apply to this deck:
If the letter is 'E', then the number on the other side is '4'.
I then drop the 4 cards on the table, and you see:
'B', '7', 'E', '4' (on the respective cards).
Which of the 4 cards must you turn over to verify or disprove my hypothesis?
Well, you did not tell us that letters and numbers could only be used once. So, just because you see an E and a 4 does not mean the opposites sides of either of those two cards does not have the other one on it. Therefore, you would have to flip over the card with the E on it and the card with the 4 on it to see if the E has a 4 on the other side or if the 4 has an E on the other side.
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