Alan Turing's 1952 reaction-diffusion model: two chemicals (activator U, inhibitor V) interact.
U self-activates and activates V. V inhibits U. Crucially, V diffuses faster than U (D_v >> D_u).
This "local activation, lateral inhibition" breaks symmetry spontaneously, creating stable spatial patterns.
Different (f,k) parameters select spots, stripes, or maze-like labyrinths — explaining leopard spots, zebrafish stripes, and coral patterns.