A prize lies inside exactly one of three closed boxes. Box A is marked 'The prize is not in this box', Box B is marked 'The prize is in this box', and Box C is marked 'The prize is not in Box B'. Exactly one of these three markings tells the truth. Where is the prize?

Try each box in turn and count the true markings. If the prize is in A: A's marking is false, B's is false, C's is true — exactly one, which fits. If the prize is in B: A's is true and B's is true, already two. If the prize is in C: A's is true and C's is true, again two. Only 'the prize is in Box A' leaves exactly one true marking, so that is the answer. The trap is Box B, chosen by trusting the one marking that names a box outright instead of testing all three cases.