You are lost in the woods and the path you are following forks into two: a path to the left and one to the right. One path will lead you to safety while the other will cause you to be lost forever.
At the fork are two twin sisters who know which path is which. The sisters are identical in every way except one: one of the sisters always tells the truth while the other always lies.
You can ask only one question and you don’t know which sister is which. What can you say to them so you know which path to take?
Ask one of the sisters which path their sister would tell you to take.
Let’s say the left hand path is the correct one to follow. The sister that lies knows their truthful sister would tell you the left hand path, so as they always lie they will tell you the right hand path.
The honest sister knows their lying sister will tell you the right hand path and because they’re honest, they will tell you this. So you should follow the opposite path to that which you are told, regardless of which sister tells you.
There are two people in front of you holding guns.
One is your best friend and the other is an impostor who looks just like him and knows all that he knows.
You also have a gun and must shoot the impostor, if you get it wrong, the impostor will shoot you. If you get it right you and your friend are safe.
What is the one question you can ask that will reveal the impostor?