Bak–Tang–Wiesenfeld sandpile model on an inclined surface. Grains are added at the top; when slope exceeds the critical angle, avalanches cascade down — self-organized criticality in action.
BTW (1987): the sandpile self-organizes to a critical state without tuning. Avalanche sizes follow a power law P(s) ~ s^(-τ) with τ ≈ 1.0–1.5 (dimension-dependent). The histogram (right) shows the distribution of avalanche sizes on log-log scale.