Grains drop one at a time onto random cells. A cell that reaches 4 grains topples, sending one grain to each of its four neighbours. Grains that fall over the edge are gone. No cell knows anything about the table as a whole.
Tap to pour 40 grains on one spot. The same ?seed= always gives the same run.
LITThe topple rule is Bak–Tang–Wiesenfeld (1987). Every 120 steps the page checks that grains on the table = grains dropped − grains lost and that no cell is left holding 4 or more, and it throws if not.
AMBERThat a power law emerges is what this page shows you; it is not proved on the page.