Cyclic Cellular AutomatonEvery cell holds a number on a colour wheel and can only ever tick forward to the next colour — but only when enough neighbours are already there. From pure random noise the field first clumps into blobs, then throws out rotating spiral waves that pave the whole screen and never stop.David Griffeath (nicknamed “Digital”) studied these in the 1990s. The rule is a caricature of any excitable medium — heart tissue, a slime mould, the Belousov–Zhabotinsky reaction — and it shows how a cyclic chase with a threshold is enough to spontaneously wind up spirals.