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

Let’s break down what happens:

Case 1: You’re speaking to the truth-teller

  • They will honestly report what the liar would say.
  • Since the liar always lies, the answer the truth-teller reports is false.
  • So, you go to the opposite door of what they indicate.

Case 2: You’re speaking to the liar

  • They will lie about what the truth-teller would say.
  • The truth-teller would point you to the correct door; the liar lies and points to the wrong one.
  • Again, you go to the opposite door.

In both cases, reversing the answer leads you to the correct door.

Final Answer

Ask either guard:
“If I asked the other guard which door leads to the job offer, would they say this one?”
Then go through the opposite door of what they say.

Reference