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[Zendo](https://boardgamegeek.com/boardgame/6830/zendo) is the logical thinking game of all time. i don't think there's anything that comes close. it was [invented or discovered](http://www.koryheath.com/zendo/design-history/) in 2001 by the late Kory Heath ([GNU](https://wiki.lspace.org/GNU_Terry_Pratchett)).
## Rules
Zendo is a semi-cooperative game played with one "master" and one or more "students". The master constructs two structures, or "koans", and chooses a secret rule, or "buddha nature". Koans that match the rule are marked with a white "true" stone, and koans that do not match the rule are marked with a black "false" stone. The students win when they can state the rule themselves, this is called "enlightenment."
For example, when playing with [icehouse pyramids](https://looneypyramids.wiki/), the rule might be "a koan has the buddha nature if it contains any red piece".
(TODO: picture of two koans that illustrate this sample rule)
## The Student's Turn
Students take turns, starting from whoever won the previous game, or on the master's left. On a student's turn they do these three things in order:
- create a koan
- say "tell me" or "quiz time"
- (optional) solve the puzzle
the student always begins their turn by creating a koan. they may take any number of pieces from the supply and arrange them in any way they like. the master must wait to react until the student is finished, so as not to spoil anything. when the student is done building, they will say either "tell me" or "quiz time".
if the student says "tell me" then the master takes a new white or black stone and marks the truth value of the new koan. the master should avoid giving any commentary that is not requested. if the student says "would you say this red piece is above the blue one?" then the master is allowed to answer, but if the student does not ask, the master should remain silent.
if the student says "quiz time" then the master judges the new koan and decides if it is true or false. the master takes BOTH a white and black stone, hides them behind their back or below the table, and then holds out a closed hand containing the correct answer. all students now do the same, taking one of each stone and holding out the one they believe is correct. when the master opens their hand, all students reveal their prediction, and any student who predicts correctly is rewarded with a "guessing stone" of a third color. the master should now complete their turn by placing the white or black stone next to the new koan.
after telling or quizzing is completed, it is still the student's turn! if the student has *any* guessing stones, they may spend one to solve the puzzle. the student says the rule out loud: "a koan must contain at least one red piece." if the student is correct, the master responds "you are Enlightened" and the game is won.
if the student solves the puzzle, and they are not correct, then the master should check to see if there is evidence already on the board. if the master can point to an existing koan that disproves the student, then the student is permitted to take their guess back and try again. if, however, the student's guess is incorrect, and there is no evidence that proves this, then the master will take the guessing token away from them and, in exchange, the master constructs a *new* koan that disproves the student's proposed rule.
(TODO: picture demonstrating a disproved rule)
after the solve attempt is completed, it is still the student's turn. if they have any guessing stones, they may repeat the above process. this is always optional: a student may guess once, or twice, or not at all. turn now procedes to the next student clockwise.
## The Master's Intervention
if the game has been going for quite some time and no student is enlightened, then the master should, at their discretion, give the students three options.
- continue as normal
- change to "free mode"
- resign
the players should choose together, with the master acting as moderator. if the players choose to continue as normal, then the master should direct play to the next student in turn order. if the players choose "free mode", then the master should give everyone infinite guesses and relax the turn order (see Variants). if the players resign, then the master must reveal the solution to the puzzle. if the players cannot all agree on an option, the master should choose for them (usually resign).
## Designing a Secret Rule
the rule must only be about things that are contained within the koan, such as the number of its pieces and their relative directions. common properties that might be in a secret rule include
- number of pieces
- size/value of pieces (1, 2, 3)
- color of pieces
- whether a specific piece is pointing at another
- whether a piece is upright, flat, or "weird" (neither)
- whether a piece is "grounded" or "ungrounded" (touching the playing surface or not)
- whether a piece is above, inside, or on top of another piece
- the total number of pieces with some property
- the total value of pieces with some property
some things that *may* be in a secret rule, but you should use them sparingly, especially with new students:
- references to complex numeric concepts like "if the sum of its values has exactly three prime factors"
- three or more clauses, e.g. "it contains a red piece AND it has an even piece count AND there are no upright pieces"
some things that *must never* be in a secret rule:
- references to specific players, e.g. "who built this koan"
- references to real-world time and space, e.g. "if it was built after noon" or "if it is pointing north"
rule-specific definitions are allowed! for instance the rule might define "a spire is a stack of two or more upright pieces" and then continue "a koan is true if it contains a monochromatic spire." i would consider this definition to be an additional clause, for the "no third clause" guideline.
### Difficulty
remember this: every rule is harder to solve when you haven't seen it before!
new masters often believe a rule is "too easy" and are shocked when it takes smart students over an hour to solve it. please low-ball your difficulty!
in general, rules with two or more clauses are medium hard, and rules which require comparing two or more variables are very hard.
you can adjust difficulty on the fly by giving more or less helpful response koans during a solve attempt. a helpful response koan is often extremely similar to an existing koan, with one key detail changed. but if your students are not having fun, never be too proud to resign and reveal your rule.
## Variants
### Free Mode
in certain situations, keeping score is not fun or viable. in these situations i recommend giving the students full control to build, solve, and talk amongst themselves. the master should still respond to any "tell me" or "solve the puzzle" requests. the master may encourage the students to build koans, especially if they have an interesting solve attempt, for instance they might say "before i answer that, would you like to test your hypothesis?" but ultimately the master should take a facilicator role here and answer whatever questions the students may have.
if playing in a virtual setting, or with a large group, it's easy for some quieter students to feel "drowned out" in this mode. if the master observes this happening, they should require students to raise their hand and wait to be called on, and make an effort to call first on those who have spoken the least.
if playing asynchronously (on a forum or message board), some students may show up late and want a chance to solve the puzzle themselves, even if another student has already achieved enlightenment. to honor these students, all guesses should be made secretly (via spoiler tags or similar), and the master should indicate only whether the guess was correct or incorrect. personally, i like to use a 'cloud watch' system: i will mark hidden guesses with ☁️ cloudy if they are incorrect, ⛅ partly cloudy if they are incorrect but require a new koan, and ☀️ sunny if the student has become enlightened.
### Graph Zendo
sometime around the start of 2020 i decided that i wanted a version that i could show to my coworkers, who i was only seeing thru video calls [for some reason](https://en.wikipedia.org/wiki/COVID-19_pandemic). but hey, y'know what every remote office solution has?
flowcharts! how could we possibly communicate if we didn't have some sort of digital equivalent of a whiteboard full of shapes and arrows, right? so with that in mind, I present to you: Graph Zendo!
## graph zendo rules summary
- a **koan** is a **graph** which is a collection of nodes and arrows.
- a **node** is a triangle, circle, or square.
- an **arrow** is a one-way path that begins at a node and ends at a node. arrows are considered to be shapeless; the rule cannot refer to "bent arrows" for instance.
- each graph should also be given a number for reference. the number is not part of the graph.
- in free play, a student can place a dot under their graph to indicate "tell me". the master should convert this dot to a 🎯 bullseye if it is true, or a ❌ cross if it is false.