Hey there, fellow puzzler. Stop me if you’ve heard this one before:
Ah yes, the riddle of the two guards. Also known as the two doors puzzle, the two guards/two doors puzzle, and, my personal favorite, that awful crap riddle.
It’s easily the most famous example of a Knights and Knaves logic puzzle, which all work off the idea of individuals who always lie or always tell the truth, and a solver who needs to deduce how to proceed.
I think the best and most colorful presentation of the riddle was in the film Labyrinth, thanks to Jennifer Connolly and some very lovely puppetry:
I’ve always wondered about this riddle. Is there a sign posted that explains the rules? Because otherwise, you’d assume the guard who tells the truth would have to explain the rules…
Unless the one who lies tells you the rules, which are a lie, and it’s a trap and you’re totally screwed!
Geez, this is getting complicated already.

Let’s assume the rules are fair. There’s a guard who always lies and a guard who always tells the truth, and you need a single question to root out which door is the safe one to enter.
The traditional answer is to ask one guard which door the OTHER guard would say leads out.
If the guard you asked is the one who lies, then he will lead you astray by telling you (falsely) that the other guard—the truthful guard—will point you towards door B. This makes door A the safe bet.
If the guard you ask is the one who tells the truth, he will tell you (honestly) that the other guard—the lying guard—will point you towards door B. This makes door A the safe bet.
In both cases, the outcome is the same: walk through the opposite door.
Naturally, people have tried to find ways around this.
Some suggest that you ask the guards to walk through their respective doors and report back what they see. I don’t know that this would work, because it’s not clear if the guards would have to follow your orders. Also, we don’t know if certain doom for US would be certain doom for the guards. If they both walk out totally fine, it probably won’t work.
For another option, you can act in typical Dungeons and Dragons-fueled fashion and use what I call the direct approach:
But unfortunately, it misses the meat of the riddle. It’s not just knowing which guard lies and which guard tells the truth, it’s about knowing which door to take as well.
Of course, if you believe the folks at XKCD, there are consequences for cleverness as well:
Although it is a riddle easily given to weasel words and pedantry, it has led to some wonderful comedy.
This joke, for instance, gives us some much-needed backstory to the guards and their current circumstances:
In a similar vein, this one lets us see one guard’s life after clocking out for the day:
I also enjoy this one, which ponders what sort of person would employ a riddle like this as a security measure in the first place:
So I put it to you, fellow puzzler: how would you tackle the riddle of the two guards? Labyrinth style? Barbarian style? Or have you conjured up a different question to ask the guards?
Let us know in the comment section below. We’d love to hear from you!






























