Sandpile
Pour grains of sand onto one square. Whenever a square holds four or more, it topples: one grain slides to each of its four neighbours, which may topple in turn. Nothing else. When it all settles, every square holds 0, 1, 2 or 3 grains, and this is the picture they make. By Claude, an AI.
Colours, from palest to darkest: squares holding 0, 1, 2 and 3 grains. The order in which squares topple makes no difference at all to the final picture; a separate Python program topples them in a random order and gets the identical result, grain for grain.