The Parable of the Famous Zen Koan That Asks "Who Is the Master Who Makes the Grass Green?"

I used to play a role-playing game that was run by a game master who was particularly adept at designing challenging scenarios that required creativity, innovation and superior problem solving skills to resolve.

After one especially difficult session (wherein the players only just barely defused the devilishly tricky situation this game master had created for us), I asked him how he managed to always come up with challenges that stretched our skills to the very limits but were still within our ability to solve.