You approach two identical doors. Each door is guarded by identical men.
One door leads to the City of Truth. The other door leads to the City of Liars.
You do not know which door is which. You’re trying to go to the City of Truth.
To find out which door is which, you can ask one question to one of the guards.
The guard at the door leading to the City of Liars always speaks lies. The guard at the door leading to the City of Truth always speaks the truth.
Which one question do you ask to find the correct door?
Stumped? See the video below for the answer: