Riddle: There are two doors. One leads to happiness and one leads to death. There are two guards. One guard guards each door. One guard always lies. One guard always tells the truth. If you had once chance to ask one of the guards a question (remember, you don't know if he is the truth telling or lying guard or what door he's guarding) to figure out what door you should enter to get to happiness, what would the question be?
Answer: The question should be, â€œIf I were to ask the other guard what door led to happiness, what would he say?â€ Take the door that the guard DIDNâ€™T point to.
If youâ€™re confused, below are two scenarios, one where you happen to ask the honest guard and one where you happen to ask the lying guard. For these explanations, the truth telling guard will be called T and the lying guard will be called L.
Scenario #1 (you ask T): T would point to the door that leads to death, because he is telling the truth. If you were to ask L what door led to happiness, L would lie and actually say the door that led to death, so since T always tell the truth, he will truthfully tell you what L would say.
Scenario #2 (you ask L): L is a bit more complicated. But we will start...backwards. If you were to ask T what door led to happiness, he would point to the door that actually led to happiness, because he is honest. So if L was honest too (which he was not) he would actually point to the door that led to happiness if you were to ask him the question. But heâ€™s not so therefore he would lie and say the door that T actually WOULDNâ€™T point to.
So, in Scenario #1 and Scenario #2 (the only possible scenarios), you should take the door that the guards do not point to.