You face a classic logic puzzle:

Two doors stand before you:

  • One leads to your dream job offer.
  • The other leads to certain doom (rejection and exit).

In front of each door is a guard:

  • One always tells the truth.
  • The other always lies.

You may ask one yes-or-no question to one guard.
You must then choose a door.

The Challenge

With only one question and no way to know which guard is truthful, how can you guarantee selecting the correct door?

The Classic Logic Trick

Ask this question to either guard:

“If I asked the other guard which door leads to the job offer, would they say this one?”
(Point to one of the doors.)

Then:

  • If the guard says yes, go to the other door.
  • If the guard says no, go to the door you pointed at.

Why It Works

You are asking a nested question. By asking one guard what the other guard would say, you guarantee that the logical path passes through exactly one truth-teller and one liar.

Because True × False = False, the final answer you receive will always be a lie.

Let’s trace it:

  • If you point to the Job door: The true answer is “Yes”. The liar would lie and say “No”. The truth-teller would honestly report that the liar would say “No”. Thus, both guards will say “No”.
  • If you point to the Doom door: The true answer is “No”. The liar would lie and say “Yes”. The truth-teller would honestly report that the liar would say “Yes”. Thus, both guards will say “Yes”.

In both cases, the “Yes” or “No” you receive is exactly backward.

Final Answer

Ask either guard:
“If I asked the other guard which door leads to the job offer, would they say this one?”

  • If they say “Yes”, go to the other door.
  • If they say “No”, go to the door you pointed at.

Reference