The pattern-making process behind leopard spots, zebra stripes and coral. It is the Gray-Scott model, and it involves two imaginary chemicals.
Chemical A is fed steadily into the whole surface. Chemical B is drained away at a steady rate. And where they meet, B consumes A and turns it into more B, so B is self-reproducing. Both spread out into their surroundings as they go, but importantly at different rates. That competition, one chemical reproducing while the other spreads faster, is enough on its own to produce spots, stripes, labyrinths and things that look like they are growing.
feed and kill are the two dials that matter, and the whole variety of the model lives in a surprisingly narrow range of them. Small changes move you from drifting spots that divide like cells, to worms, to a solid maze. It is worth moving them slowly. Outside a fairly small window the pattern either dies out completely or floods and fills.