You are walking down the street, where you see a cross road. You wanted to know directions for the place you wanted to be. There is a left path and a right path. You suddenly see a house nearby, and said that maybe someone would give me the right direction. 2 brothers live in the house. One who lies on everything, and the second who says the truth. You don't know who is gonna open the door to tell you directions.
You have one question to propose. This question should lead you to the right road whether the lier opens up, or the trustworthy one.