Are you guarding this door?
But you don't know which guard is guarding which door. Your question will tell you which guard you are talking to but not which door he is guarding.
The guard who has to lie knows you will ask a question so his lie is the truth which is a lie on his requirement.
Good point bill.But you don't know which guard is guarding which door. Your question will tell you which guard you are talking to but not which door he is guarding.