You come to a fork in the road. One road leads to a diseased village, the other road leads to a great amount of riches. On both sides of the road there is a person. One is a pathological liar (everything he says is false), the other a pathalogical truth teller (everything he says is true). If you get to ask a single question to one of them, what would you ask to choose which road you will continue your journey on?
You ask one of them what the other guy would answer if you asked for the path to riches. They should both point to the path that leads towards disease so you take whatever path they don't point to.