This one's a classic. You are traveling alone, when suddenly, the road splits. You know that one of the paths is the good one; the other one will lead to death and destruction. Alas, you have no tools to guide you, but luckily, there is a guard at the start of each path. One of the guards always speaks the truth, the other one will always lie. Unfortunately, you don't know which one is which.
Efficient as you are, you only want to ask one question to either one of the guards to know the right path and be on your way.
What question will take you on the right track?
The main trick is to take advantage of the mutual answer of the two guards; this will always lead you to the wrong path. For example: "what path is the right one, according to that other guard over there?"