Four cards lie on a table. Every card has a letter printed on one face and a number on the other. The faces you can see read A, K, 4 and 7. Someone claims: 'On every card that has a vowel on one face, the other face carries an even number.' Which cards must be turned over to find out whether the claim is false?

Turn A: it is a vowel, so an odd number behind it would break the claim. Turn 7: it is odd, so a vowel behind it would break the claim. The 4 need not be turned — the claim says nothing about what may sit behind an even number, so neither a vowel nor a consonant there could break it. K need not be turned either, since the claim is only about vowels. The trap is 'A and 4': turning 4 looks helpful because it hunts for an example that agrees with the claim, but a claim can only be knocked down by hunting for the case that would break it.