Imagine a child on the beach letting the sand slowly flow down to form a sand pile on the ground. At first the sand pile was flat, and each grain of sand was located near its landing point, and their transport was carried

Imagine a child on the beach letting the sand slowly flow down to form a sand pile on the ground. In the beginning the sand pile is flat, each grain of sand is located near its landing point, and their movement can be explained by their physical properties. As the sand continues to flow down, the sand pile becomes steeper and small sand-grained landslides form. Over time, the sand-grained landslides gradually become larger, and eventually some sand-grained landslides may even cover the entire sand pile. At that point the system is no longer in equilibrium", and its phenomena can no longer be understood in terms of the motion of individual grains of sand. Sand-grain landslides form their own dynamics of motion, which can only be understood by describing the overall nature of the sand pile, but not from the original nature of individual sand grains: the sand pile is a complex system.

(When the pile of sand is raised to a certain extent, falling a grain of sand may cause the entire pile to collapse.) At this point, the sand pile is in a "self-organized criticality" state. When the sand pile reaches a critical state, each newly fallen grain of sand will pass the collision sequence to all the sand grains, resulting in a holistic chain change of the sand pile, and the sand pile becomes more and more fragile, and eventually collapses. ）

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