Mao (also sometimes called Mü, Mile, Chairman, Dictator, Bjorn, Maul, Maui, or King Mao) is a card game. Since it is forbidden to say Mao's rules, new players are often told only "the only rule you may be told is this one." As such, the rules of the game are discovered by playing or watching the game. A player who breaks a rule is penalized by being given an additional card from the deck. The person giving the penalty must state what the incorrect action was, but must not say the rule itself.
As one might suspect, with a game of this nature, there are many variants of Mao in existence. While beginners sometimes complain that the dealer and other experienced players are simply making up possibly inconsistent rules (as in the game Mornington Crescent), the rules of Mao should be consistent within each game and can be followed correctly.