a falling-sand toy, the kind that was everywhere in the mid-2000s. the black box is a grid of cells and every frame each cell follows one dumb rule based only on what is touching it. fall if you can, slide diagonally if you cannot, react to your neighbors. that is the whole engine. pick a material, scribble in the box, drop lava on water, or set a plant on fire and watch it run.
built from scratch, no libraries, ~200 lines of physics in one file. full write up of how it works, the numbers, and the bug that ate an hour is in notes.md. code: github.com/xevrion/falling-sand.