Yes, thank you!! Found the scene itself since the whole serial is apparently on youtube: https://youtu.be/lLBHbt9QYFU?t=5458

Funny how my memory had it in black and white. And I remember the scene being much longer. I watched it when I was like, 9.

The first time I encountered a version of this riddle it actually wasn’t Labyrinth. It was an old black and white episode of Dr Who aired on PBS when I was a little kid. Same scenario but if I recall, robots instead of guardsmen. I think the good doctor solved the riddle in the typical way of asking one robot what the other would say. I’m looking for it now but I can’t find the scene.