You're standing at a crossroads and don't know the correct path to take. There are two men standing at the junction who could possibly help. The thing is...one is a truthful man and the other is a liar. You are only allowed to ask each man 1 question, but it has to be the same one. What then, do you ask? Hint: even after you've figured out the correct path, you still won't know which man is the liar! Have fun.
(ask the other doorkeeper) tell me this door leads to the castle? If the door she was indicating was the wrong one and she was asking the lying doorkeeper, his answer would be yes - because he would lie. If she was asking the truthful doorkeeper, he too would answer yes - since the other doorkeeper would lie. In either case, the other door would be the correct one. If the door she was indicating was the right one and she was asking the lying doorkeeper, his answer would be no - because he would lie. If she was asking the truthful doorkeeper, he too would answer no, since the other doorkeeper would lie. In either case, this would be the correct door