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, but the guards do know.
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?