Calvinball is a super fun and crazy game that two people invented in a comic book called "Calvin and Hobbes." In this game, there are a lot of rules, but the catch is you get to make up new rules as you go along!
When you start the game, there are no rules, but as soon as someone does something, they get to make a new rule. This means the game can get really silly really fast! You might have to hop on one foot only, or sing a song before you can catch the ball, or even wear a silly hat while you play.
The great thing about Calvinball is that nobody really knows how to win, and nobody really cares! It's all about having fun and making up new rules as you go. So, if you want to play Calvinball, all you need is a ball, a friend, and lots of imagination!