Grains rain randomly. The system self-organizes to a critical state where avalanche sizes follow a power law: P(s) ~ s^(-τ) with τ ≈ 1.2
Power-law: straight line on log-log plot
Collecting data...
The BTW sandpile reaches a critical state where adding a single grain can trigger avalanches of any size. This is self-organized criticality -- the system tunes itself to the critical point without any external parameter tuning.