Suppose the coinage of a country has a portrait of one of its eminent sovereigns on one side and a specimen of its magnificent fauna on the other. Now consider a simple if-then rule: “If a coin has a king on one side, then it has a bird on the other.” Here are four coins, displaying a king, a queen, a moose, and a duck. Which of the coins do you have to turn over to determine whether the rule has been violated?

Answer: If you’re like most people, you said “the king” or “the king and the duck.” The correct answer is “the king and the moose.” Why? Everyone agrees you have to turn over the king, because if you failed to find a bird on the reverse it would violate the rule in so many words. Most people know there’s no point in turning over the queen, because the rule says “If king, then bird”; it says nothing about coins with a queen. Many say you should turn over the duck, but when you think about it, that coin is irrelevant. The rule is “If king, then bird,” not “If bird, then king”: if the duck shared the coin with a queen, nothing would be amiss. But now consider the moose. If you turned that coin over and found a king on the obverse, the rule “If king, then bird” would have been transgressed. The answer, then, is “the king and the moose.”

— Steven Pinker, Rationality