You are a prisoner in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a hangman–you don't know which is which.
One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either. However both guards know each other.
You have to choose and open one of these doors, but you can only ask a single question to one of the guards.
What do you ask to find the door leading to freedom?