You are walking along the road when you reach a fork, where you see two guards chatting with each other. You approach them to ask which way you should go, but you notice a peculiar sign that says: “One of us always tells the truth, one of us always lies”. There is no way for you to know who tells the truth and who tells lies. If you could only ask one question to help you walk down the right path, what would you ask the guards?
The solution is simple: all you have to do is ask “If I asked the other guard which road is the right one, which road would he tell me to take?” then go to the opposite road. The reason being, no matter who you ask – truth-teller or liar – they will tell you the wrong answer. If you asked the truth-teller, he would honestly reply with what the liar would say, which is the wrong answer. If you asked the liar, he would tell you the opposite of what the truth-teller would say, which would ultimately be the wrong answer.