Sooner or later, all games turn into Calvinball

Not sure I have ever heard of Calvinball before. According to this fan site,

Calvinball is a game invented by Calvin and Hobbes. Calvinball has no rules; the players make up their own rules as they go along, making it so that no Calvinball game is like another.

Rules cannot be used twice (except for the rule that rules cannot be used twice), and any plays made in one game may not be made again in any future games. The game may involve wickets, mallets, volleyballs, and additional sports-related equipment.

There is only one permanent rule in Calvinball: players cannot play it the same way twice. For example, in one game of Calvinball, the goal was to capture the 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. Masks must be worn at all times in Calvinball; these are not allowed to be questioned.

Calvin created the game when he first became tired of organized sports. Although the first depicted game of Calvinball followed Calvin's failure to join the baseball team, the game appears in such a complete form there that it is likely that Calvin and Hobbes had been playing the game for a long time beforehand.

This observation seems topical and almost profound. And true.

The only hint at the true creation of the game comes from the last Calvinball strip, in which a game of football quickly changes into a game of Calvinball. Calvin remarks, "Sooner or later, all our games turn into Calvinball," suggesting that a similar scenario led directly to the creation of the sport.