The Bak-Tang-Wiesenfeld sandpile is the paradigm of self-organized criticality (SOC).
Grains are added randomly; when a site has ≥4 grains it topples, distributing to neighbors.
No parameter tuning is needed — the system self-organizes to a critical state where
avalanche sizes follow P(s) ~ s^{-τ} with τ ≈ 1.2 (2D).