From Looney Pyramid Games Wiki

In Zendo, a Mondo is a challenge to all the players at once, testing each one's understanding of the secret rule.

A player begins by building a koan, then calling "mondo". All players do their best to determine whether the koan has the Buddha nature, simultaneously revealing their choice of marking stone. Each student who answers the mondo with the correct white or black marking stone receives payment of a token, but there's normally no penalty for guessing wrong. Earned tokens (often green stones) may be used at the end of a player's turn to purchase guesses at the secret rule.