Zendo is an inductive reasoning game by Kory Heath. Heath reworked Eluesis, a game designed to play with a standard 52-card deck by Robert Abott. Eluesis was first published in Scientific American, June 1959, but has been published several times since in card game rulebooks. Heath created a complete boxed set with colorful transparent pyramids, guessing tokens, sample puzzles, rules and a new Buddhist theme rather than the original Judeo-Christian theme. Zendo is more colorful and tactile because it uses 3D pyramids rather than standard cards.
It's an inductive reasoning game because you are given specifics and must work thorough to find the general rule that produces the specifics. In inductive reasoning, you follow a course from observation > hypothesis > theory, while deductive reasoning flows in the opposite direction: theory > hypothesis > observation.
Before play, a Master (puzzle maker) is chosen and all other players become Students. The Master creates and secretly writes down the rule or chooses one from a sample rule deck. The Master than creates two Koans (experiments) that are true and false to the secret rule. A Koan is a specific arrangement of pyramids that either follow (true) or break (false) the secret rule.
A black stone is put on the Koan that breaks the rule and a white stone is put on Koan that follows the rule. The Students then take turns. A Student first builds a Koan. The student then declares Master or Mondo. When Master is declared, the Master will mark the new Student's Koan with a black or white stone. Declaring Mondo will make all Students guess simultaneously by holding out a black or white stone. A correct guess is rewarded with a green guessing stone.
Finally the Student has the option of guessing the rule, by paying one green stone. If the rule that the Student offers is wrong, then the Master creates a new Koan that follows the Master's rule but disproves the Student's guess. If however, the Student guesses correctly, then the Student wins the game.
The pyramids come in four different colors and three sizes, are hollow, and have pips on them. You can get creative and use any aforementioned features, orientation, placement of the pyramids, to create rules. A simple rule might be, "... have a piece point to another piece." A nearly impossible rule to figure out that Board Game Geek user Ron Laufer mentions is "...if and only if the sum of the pips is a prime number."
What may be interesting for game designers is that playing Zendo teaches the art of creating the golden median of puzzles -- not too easy and yet not impossible. The reason that a deck of sample rules is included in the box is because most beginners make unrealistically hard puzzles. Beginners wrongly think that what is obvious for them must be easy for others.
Zendo is out of print, but as Kory Heath outlines on his game page, you can get all the components of the game separately to build your own set. Variants of Zendo are played on forums like on Board Game Geek, using text, icons or images rather than physical pieces. You can also practice with Zendo-San, a fanware AI by William Shlaer, that plays a more restrictive variant because the app lacks real 3D modeling.