Calvinball, according to Wikipedia
Calvinball is a game played almost exclusively by Calvin and Hobbes as a rebellion against organized team sports (like baseball), although the babysitter Rosalyn plays on one occasion. Participants of Calvinball wear masks; when asked why, Calvin replies that "no one is allowed to question the masks." The rules of the game, besides that a soccer ball and wickets are almost always used, are invented as they go along, but one consistent rule is that the rules can never be the same twice (which in itself is a self-denying paradox). Either player may change any rule at any time, so the only way to break the rules is by using one rule twice. Scoring is also entirely arbitrary: Hobbes has reported scores of "Q to 12" and "oogy to boogy." Calvinball is essentially a game of wits and creativity, rather than purely physical feats. However, it's a running joke that Hobbes is typically more successful at the game than Calvin himself. Calvinball could be described as a Nomic game, and thus bears a similarity to others such as Mornington Crescent and Stanley Random Chess.