A mutual exclusion problem...
You stay at a cross and don't know which direction you have to go.
Either to right or to left. You meet two people.
One person always says the truth, the other person however always lies.
You don't know which person says the truth or which person lies.
You have only one chance to ask one of the person a question to find the right way.
How would you form this question? Which direction would you go?
