I was once teaching it at a meetup group and an observer said 'So it's like Mastermind with Tetris pieces?' and I turned to her and said 'it is EXACTLY like that'
Zendo belongs to the school of deduction games where one person comes up with a code or rule and everyone else has to figure it out. Mastermind is the defining game of the genre but Zendo blows every other game of its type out of the water.
One player will take the role of code maker or master. Everyone else will be their students or code breakers. The master will come up with a rule about the Buddha and then build two dioramas called koans out of the pyramids , one that demonstrates the rule and one that does not.
The rule really can be virtually anything. It can relate to the colors of pyramids, the number of pyramids, the orientation of pyramids, the size of the pyramids or any combinations of them. For instance, a rule could be that a koan has the Buddha nature if it has a large blue pyramid pointing up.
Students take turns building koans of their own and then asking the master either master or mondo. If they ask master, the master simply marks if the koan follows the rule or not. Mondo has all the students guess what the master's answer will be before the master marks the koan. Everyone who guesses right gets a guessing stone.
At the end of a student's turn, they can turn in a guessing stone to guess the Buddha's nature. If they get the rule right, they win. Otherwise, the master will build another koan that demonstrates the student's rule but still does not have the Buddha's nature.
Zendo is an amazing gaming experience, unlike anything else I've played and I've played a lot. While there is a 'winning' condition and it can be seen as competitive, I've always found it turns into a cooperative experience. And unlike cooperative games like Pandemic, it isn't about tension but exploration and discussion.
The master's goal isn't to stump the students. Trust me, that wouldn't be a challenge. The master's goal is to challenge and engage the students. The students are going to inevitably work together, whether they intend to or not since every turn gives new information. In almost every game I've played in, it has become intentional.
Zendo is really about communication. A game of Zendo is a conversation between the master and the students, as well as the students with each other. That gives the entire game a vibe that is unique.
You can play Zendo with just about everything. I've read about people using pocket change to play it or words or even emoticons. However, the pyramids are a very good way to play Zendo. They create a striking visual look. More importantly, they create a distinct language to have the conversation with.
Zendo is an impressive game. Each piece of the game is designed to help people not just explore reasoning but communication. Other deduction games, like Clue or Sleuth, are about find answers. Zendo is about talking to each other.