The Abelian Sandpile Model (ASM) is a classic example of a self-organized criticality system.
How it works
Each grid cell holds an integer number of grains.
When a cell reaches the threshold, it topples, sending one grain to each orthogonal (up, down, right, left) neighbor.
The final pattern is independent of toppling order (Abelian property).
Click the canvas to add grain sources; multiple sources will interact in interesting ways.
You an also play with the threshold and colors, just click on controls to find all you need.
And click on inspiration for a real deep dive, if you are up for it..