Abelian SandpilePour a huge heap of sand onto one point. Whenever a square holds four grains it topples, one grain to each neighbour, setting off chain-reaction avalanches. When the dust settles you get this fractal — and, remarkably, the same picture no matter which grain you happened to topple first.Bak, Tang and Wiesenfeld built the model in 1987 to explain why nature so often sits right at a tipping point — earthquakes, forest fires, market crashes — without anyone tuning it there. Mathematicians then found the pile is an abelian group: the toppling order genuinely does not matter, and the limit shape is still not fully understood.