If you ever studied logic, you have definitely seen this question. I found it in Enderton though I'm not sure that it is originally from there.
You are in a land inhabited by people who either always tell the truth or always tell falsehoods. You come to a fork in the road and you need to know which fork leads to the capital. There is a local resident there, but he has time only to reply to one yes-or-no question. What question should you ask so as to learn which fork to take?
An easier question is: if you just wanted to figure out whether the resident was a liar or not, what would you ask?