You walk up to a mountain that has two paths. One leads to the other side of the mountain, and the other will get you lost forever. On the road you meet two twins that know the path that leads to the other side. You can ask them only one question. One of the twins only tells lies and the other only tells the truth, and you don't know which is which.
So, What do you Ask?
Which way would your brother say?
Example: If the good path is left, and you ask the liar this question, he'll say Right, because his brother will say left and he himself lies. The truthful fellow will say right, because he tells the truth, and the other one lies.