Three discs are inset into the contraption in the wall, each with a gnomish rune: M, F, G. They are able to be arranged within the holding areas at either end of a line cut in the wall. A disc may only pass along the line one at a time when enveloped in a third device.
Inscribed inside the inset triangle at the center of the room in gnomish is the puzzle:
“On a subterranean stroll with my three children, Myta, Fuil, and Gead, we encountered a narrow bridge they were too frightened to cross alone. I considered it only safe to cross with one at a time, but Fuil always fought when left alone with either Myta or Gead. How did we cross without a fight breaking out?”
In an unbreakable crystal case are 3 keys one of which shatters each time the F disc is alone with either of the others.