Conway's Game of Life. Every cell lives or dies by the company it keeps: a living cell with two or three living neighbours survives, a dead cell with exactly three springs to life, and everything else fades. From those three lines come gliders, guns, oscillators and chaos. Draw a seed below — or drop in a ready-made pattern — then run it.
The view scales itself to the colony as it grows and shrinks; grab it with the mouse or a finger to zoom and pan by hand, and hit Auto to give the camera back its head. The world has a hard edge, but it's a tidy one: anything that sails into it leaves cleanly, with no debris left rattling at the border.
Edit (while stopped): drag to draw, right-drag to erase, pick a Stamp then click to place it ([/] to rotate). Run: Space play/pause, S step, R reset. Zoom with the wheel, pan with a middle-drag (or a drag while running); A toggles Auto. No edits once a run is in progress — Reset to your seed to change it.
Devised by John Horton Conway (1937–2020) in 1970. He was famously ambivalent about it — this little grid eclipsed the deep mathematics he prized far more, from the surreal numbers to the Conway groups and monstrous moonshine — yet it remains the most beloved thing he ever set loose. Played here in his memory.