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Calvinball, as the 6-year-old explains to his imaginary tiger friend, is an entirely made-up game.

Its remarkable to look at, really; the games are amazing spectacles.
The key to Calvinball, though, is that there are no set rules.
Anything thats done could be immediately undone.
Doing and undoing are pretty suspect guidance for whats happening, in fact.
There are no rules, so theres no defined objective.
Its really about the experience of being there.
Theres a lot to be said for Calvinball, especially as a form of imaginative play for 6-year-olds.
Its a test of creativity.
Its obviously fun for the participants.
And to some extent, it can be fun to watch as an outside observer.
Luther (Tom Hopper) is very strong, and its almost never important in the story.
Sometimes (mostly) he does nothing.
What makes it possible for Vanya to, on occasion, fly?
None of it makes any sense!
Events are highly mutable in one scene, and stubbornly fixed in the next!