whatA pile of sand fed one grain at a time. Mostly nothing happens; now and then a grain sets off an avalanche, and the avalanches come at every size, from one cell to sweeps that cross the sheet. The chart at top right collects their sizes on doubling scales: its long, straight descent is the heavy tail. Move the pointer to choose where the grains fall; click to pour a handful.

whyThis is self-organized criticality, the Bak, Tang and Wiesenfeld model: the pile tunes itself to the edge of stability with no one adjusting anything, and at that edge there is no typical avalanche size. Earthquakes, forest fires and blackouts keep statistics of the same shape, which is why the rare enormous one is not an anomaly but part of the pattern.

howEach cell holds up to three grains, drawn as up to three hatch ticks. A fourth makes it topple: it gives one grain to each of its four neighbours, who may then topple in turn, one long chain settled before the next grain falls. Grains that leave the edge are lost, which is what lets the pile find its stationary, critical slope. Each topple flashes as it happens. The histogram counts finished avalanches in doubling size bins on a log scale, so a straight line means a power law. One integer seeds the starting pile.