You are walking down a road and you encounter a fork in the road and you do not know the correct way to go. There are two men standing at the junction and both know the correct road. One man always lies, and the other man always tells the truth, but you do not know who is the liar and who is the one who tells the truth. You can only ask each man one question, but it must be the same question for both. What is the question that would allow you to get the information necessary to choose the correct path?
You ask, "Which way would your companion say is the correct path?". The liar would lie tell you that the truthful man would say to go down the wrong path. The truthful man would tell the truth and say that the liar would tell you to go down the wrong path. They both point to the wrong path, so then the alternate is the proper path.