Self-organised criticality 

A thin stream of salt is poured from above, one grain speeding after another, forming a cone shape.  Keep pouring; the movement of the growing mound is not smooth - it judders, and the friction between the chrystals makes a jostling action as the irregular shapes quickly find ways to settle against each other.

The mound builds up. There is a small quick avalanche and the salt forms a new shape which is stable; the increasing instability has pushed it into ‘finding’ a new stable form. The ‘behaviour’ of the salt cannot be understood in terms of the individual grains; the avalanches are behaving as a massed dynamic whole.

This ‘complex system’ behaviour can be found in other parts of life. Forest fires spread like this, sparks building to walls of flame; confidence in the stock exchange builds bullishly and collapses bearishly; a sneeze becomes Lockdown.  Wikipedia also includes solar flares, sociology, plasma physics and bio-inspired computers as sharing this concept of complexity.