How Sarah worked out the puzzle in the 1986 movie Labyrinth

Question

Link to the scene: https://youtu.be/grOpvXBmTx8?si=7dT8NONFH8myDLdf

This puzzle has confused my dad when it first came out and me since he first showed it to me.

In: Other

Answer 1

By asking one of the guards what the other would say, you are guaranteed to get the WRONG answer. It removes the 50/50 uncertainty.

“Is this door safe?” “Yes.” So it must be deadly.