Click anywhere on the grid to plant a spiral. Each click seeds a broken wavefront — an excited arm with a refractory stub — which curls into a rotating spiral wave. The spiral persists because its tip always finds resting cells ahead and refractory cells behind.
This is a
Greenberg-Hastings cellular automaton: three states (rest → excited → refractory → rest), one rule (a resting cell fires if any neighbor is excited). The spiral is emergent — no cell knows it's part of one.