Calvinball

Calvinball is a game featured in the comic strip Calvin and Hobbes as one of the titular pair's favorite pastimes.
 
It was introduced on May 5th, 1990, a strip in which Hobbes remarks, "No sport is less organized than Calvinball!"

Rules

According to Calvin, "The only permanent rule in Calvinball is that you can't play it the same way twice!"

This leads to almost infinite variety in the equipment, scoring, and gameplay.

Equipment

This will vary depending on whatever the rules happen to be for each particular game, or even the particular time within a given game.

A simple black mask with eye holes is standard fair, although their exact purpose is never made clear. "Sorry, no one's allowed to question the masks." - Calvin.

Flags and croquet wickets are usually incorporated in various ways as well, as are random pieces of "traditional" sporting equipment, such as rackets, volleyballs, etc. Non-sporting equipment, e.g. a stick horse, can also be used.

A field of some type is required, which may includes trees, rocks, and streams as well as more open spaces.

Most games feature various "zones", for example, "The Corollary Zone," "The Perimeter of Wisdom," "The Boomerang Zone," or "The Vortex Spot."

Scoring

Scoring is a complicated affair, as the units are not limited to simple numbers, and can include letters or any other form of measurement a player wishes. The current score at any given point in time does not necessarily have to correspond to the game in any meaningful way.

Gameplay

Because of the almost endless variety in rules, the gameplay can vary wildly. However, hitting an opponent with the Calvinball, stealing an opponent's flag, and catching an opponent doing something illegal are frequent features.

Penalties are often incorporated into the game, sometimes corresponding to different zones. In one game, after touching "The Pernicious Poem Place," Calvin is forced to bring a bucket of ice-cold water to his neighbor Susie Derkins and recite a poem in which he asks her to dump the bucket on his head. Other penalties have included being forced to catch a water balloon thrown high into the air, and having to sing the "I'm So Sorry" song.

Calvin and Hobbes love the game because while half of the game involves actually playing within the rules, the other half involves making up and changing them. In this respect, it is similar to a real-life nomic. This type of game lets Calvin take advantage of his unending creativity in ways that more organized games cannot.