Which way goes to Mecca?
It's a very classical one so I am sorry if you know it.
Anyway it's a damn hard one. If you know the answer (because you have known this one for long) please use spoiler tags. If you are guessing, feel free to post here.
A man is going to Mecca. As he is walking he comes to point where the path goes to two directions. The path he he's been following for long goes to left and right, too. There are two guards at the crossing. They know which path leads to death and which one goes to Mecca.
But!
One of them is always telling the truth.
The other one is always lying.
AND you can ask only one question from them. Only one.
What should the man ask to find out which path goes to Mekka? If course the man doesn't know which is the truth teller and which is the lie teller.