Particles interact according to simple rules but produce complex behaviour.

In p, a bunch of particles interact with each other according to this simple rule: every particle moves towards, away, or watches another particle(s). When the program starts, each particle chooses a random color, position, and the particle(s) it will move towards, away, or watch. As the program runs, some particles join together to form a train and journey together thereafter. Some orbit each other. Some collapse into each other. Some form swarms that split and join. One cannot predict when or where or how many of these behaviours will emerge but they usually do.