沙堆模型程序。一个理想实验:通过装置让沙子一次一粒均匀地落在桌上，形成逐渐增高的一小堆，借助慢速录像和计算机模仿等手段精确地计算沙堆顶部落下一粒沙会带动多少沙粒移动 初始阶段，沙子下落对沙堆整体的影响不大 当沙堆的高度达到一定程度以后，一颗沙子的落下可能引发整个沙堆的崩塌。-SandpileModel.cdf

The Abelian sandpile model, also known as the Bak–Tang–Wiesenfeld model, was the first discovered example of a dynamical system displaying self-organized criticality. It was introduced by Per Bak, Chao Tang and Kurt Wiesenfeld in a 1987 paper.[1]