Forest-Fire ModelTrees sprout at random on a grid. Every so often lightning hits one, and fire jumps from any burning tree to its green neighbours. Watch and the same thing keeps happening: the forest thickens until a spark clears a huge swathe, over and over, with no steady state.Drossel and Schwabl wrote these three rules in 1992. With rare lightning the forest tunes itself to the edge of connectivity, so burn scars come in every size at once — self-organised criticality, like sandpiles and earthquakes.