Bak-Tang-Wiesenfeld model (1987):
If a cell has ≥ threshold grains, it topples: distributes 1 grain to each neighbor, loses 4. Grains falling off the edge are lost.
The system self-organizes to a critical state — avalanche sizes follow a power law P(s) ~ s^{-3/2}, the hallmark of SOC. No tuning required: criticality emerges spontaneously.