There is only one permanent rule in Calvinball: You can't play it in the same way twice. (But another seems to be that you have to wear a mask.) For example, in one game of Calvinball, the goal was to capture your opponent's flag, whereas in a different game of Calvinball, the goal was to score points by hitting badminton shuttlecocks against trees using a croquet mallet. An apparent rule in Calvinball is that you must wear a black mask and that you aren't allowed to question the mask. Another apparent rule is that any new rule made up by each player must be accepted. A third apparent rule is that you cannot make any plays you made in a previous game.