You are walking down a road and arrive at a fork that splits the road into two separate paths. When you reach, you meet two identical twins, each standing at the edge of one of the two paths. You have heard that there is a bag with $1,000,000 at the end of one path and you are told that one of the two twins always tells the truth and the other always lies. There is no way you can get to the end of both roads before the sun goes down.
What ONE question can you ask (of only one twin) that would lead you to the money?
There are two possibilities: either you approach and ask the liar your question OR you approach and ask the truth-teller your question.
If you ask the liar “where would your twin tell me to find the money?”, he would obviously tell you to go down the wrong path (because his twin would tell you go down the correct path and the liar must always lie).
Otherwise, you would ask the truth-teller your question: “where would your twin tell me to find the money?” and the truth-teller will also give you the wrong answer because he must honestly say what his brother would answer.
Either way, whichever answer you get from either twin, you go down the opposite path.