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What is CalvinBall?

It's quite simple, really. CalvinBall is a game that you never play the same way twice. Anyone can add a rule, and not all parties must agree for a change to be implemented.
Score is kept in an unusual alpha-numeric configuration, one example could be 27-ka blooy! to 12teen&blarg.

Many people recognize CalvinBall at the creation of legendary comic strip writer/artist, Bill Watterson. It was introduced in his Calvin & Hobbes comic strip as another way to show off Calvin's extraordinary imagination.

As you can see, the rules of the game are quite difficult for an outsider to comprehend. Note the simultaneous use of equipment from at least four different sports - it's apparent that Calvin is just using whatever falls to hand.
Here is an example of spontaneous rule adjustment, which is one of things that makes CalvinBall unique:

And you can go here to add your own rules!
These don't have to be based on the comics - add whatever you like.
Remember the Golden Rule: