oriental rug rock wall spider web &#151; A large number of subunits (individuals or building blocks) and a large number of interactions Example: grains of building material with simple rules for pickups &amp; deposits &#151; The starting point is often a homogeneous, randomly distributed field &#151; Positive feedback (local activation or attraction, autocatalysis) rule: put grains nearby other grains (ant behaviors or physics of windblown sand) The logic is the same.