Riddle: There is a group of people. Half of them always lie. Half always tell the truth. The people that always lie wear only black socks. The people that always tell the truth wear only yellow socks. If you were to go up to one of these people, but they pulled off their sock before you could see the color, what one yes-or-no question could you ask them to figure out what color their socks were?
Answer: Any question that you know the answer to, for example, "Is it raining outside?". You obviously can tell if it is raining out side or not. Assuming it's not raining, if they say yes, then they always tell the truth, therefore they wear yellow socks, if they say no, then they always lie and wear black socks. But there is a very big span of questions you can ask them. But remember, a question like, "Do you lie?" Doesn't work, because both groups will say no.