This question already has an answer here:
You are on a trail in a desert. You already drank all your water, and are 2 miles from the next well.
There is a crossroads in the trail. One path leads directly to the fountain, and one leads out into the middle of nowhere. At the crossroad, there is an Indian. He is from one of two tribes - one tribe always tells the truth, and one tribe always tells lies. The two tribes have one thing in common: They will only answer one question.
What question must you ask to find out which trail to take?
I'll post the answer in a few days if no one knows it.