Zendo, by Kory Heath, is a good game. I recommend it. Go play it.
One participant creates a puzzle. The other players compete with each other trying to solve it. The puzzle-creator isn't the adversary. (The deets about how the puzzles work and terms of the player's competition with each other are beyond the scope of this post.)
The object of the puzzle-creator is not to make a puzzle so difficult that it can't be beaten. Nor a puzzle that's too easy. The puzzle-creator isn't competing with the players. She is challenging them.
Someone recently used the metaphor of puzzle-maker and puzzle-solver to explain to me something that I had thought was incoherent in trad roleplaying. The puzzle-solver can have the mindset of competition. Trying to beat the puzzle. Struggling.
The puzzle-maker has a completely different mindset. They can make an unsolvable puzzle easily. Once I played a very cruel prank on someone. We were sending puzzles back and forth. And I sent a square of random letters, numbers, and diamonds. A fake "puzzle". (The intended outtome I had in mind -- which in hindsight was unlikely -- was that the solver would realize that the only winning move was not to play, would see the Gordian cut. Of course than did not happen. Instead, it eroded trust in the implied social contract between solver and maker.) That was a mistake. I was very young, and I learned from it. The puzzle-maker's mindset is to find the sweetspot of difficulty. Goldilocks. The puzzle-maker "wins" if the puzzle is a challenge, is pleasant to try to solve, is instructive, is doable, is difficult, is possible, is fair
So that is how it is. The players "cheating" is different from the DMs "cheating". I finally heard a good argument for that. I enjoy solving puzzles. I enjoy creating puzzles. I had never connected that mentality -- those two orthogonal but sharply intersecting mentalities -- to DM and player in a trad or old school roleplaying game.
I had a mental model of a cheating DM to be similar to someone cheating in chess, someone who pretends to play chess with you but who actually just wants you to have a compelling experience. But. That's not what good chess teachers do. They neither play to beat you nor pretend-to-play. What they do is set up puzzles. Can you see the back rank mate in this situation? Can you see the distraction, the prophylactic move, the fork? This has truly been a missing puzzle piece for me to understand the trad trpg mentality.
However. However, the game of Zendo has rules for how the puzzle-maker interacts with the players and the puzzle. The puzzle consists of a secret "rule", and of "structures". The players use the structures to try to find the secret "rule". The puzzle-maker is allowed, once the game has started, to create structures that make the rule easier or harder to find. Again, with the goal of finding the sweet spot of difficulty, and trying to be fair to all players. They are, after all, competing with each other, even though they're not competing with you.
The puzzle-maker is not allowed to change the secret rule once play has started. That would break the game. It is not allowed to have no rule in mind, building new structures to disprove the players guess while continuously allowing many different variations of secret rule to be possible. It is not impossible to play the game this way. Eventually, and with great difficulty, one secret rule might become nailed down. Conversely, having no rule in mind but agreeing with the first suggestion by any player, would also break the game.
Both of these are powerful techniques, in Zendo, to achieve the puzzle-maker's goal of having the puzzle challenge be just the right difficulty and length. When I am the puzzle-maker in Zendo, I am generally happy if a particular rule goes around two or three turns around the table. If every participant first has some wrong idea of what the rule might be, but then when one player is getting close to finding what the rule might really be, so are the others. Having no rule in mind allows me to prolong the game as I wish, by continuously making the rule more difficult, and to end the game whenever I want, by declaring any valid guess -- any guess that isn't disproven by preexisting structures -- to be correct.
That, however, is against the rules of Zendo. That breaks the covenant between puzzle-maker and puzzle-solver. That… is cheating. The rules of Zendo are that the puzzle-maker does have one secret rule in mind, and sticks to it.
In D&D… I wear two separate hats. The prepper, and the runner. When I am running the game, I may not alter stats on the fly on the sly. HP, saves, number-appearing… the prepper should set those when she makes the world. The prepper makes sure the runner has a location, a living, populated location and the life, the population, has stats. Mechanics.
Similarly, in Zendo, I create a rule, trying to make that rule a good, fair challenge. I have some liberty once the game starts to build structures that are easy or hard. But I can not touch the secret rule.