Sandpile model of Bak et al.
Sand grains are sequentially dropped in the middle. Color coding:
Rule for cascading: If more than 3 grains of sand land on a site, they are distributed to the 4 nearest neighbor sites.
Withdraw sand one grain at a time from the center site. If a site has 0 grains of sand, then we withdraw 1 and draw in 4 grains from its four nearest neighbors.