Thousands of particles caught in a strange attractor, tracing out the famous butterfly shape as they go.
Three effects make the program. pointsEmit() creates the particles, attractor moves each one according to the attractor's equations, and pointsRender() draws the trails they leave. It is a genuinely three-dimensional system, so it is worth watching with pointsRender(viewMode: ortho).
The behaviour comes entirely from the equations. There is no rule anywhere telling the particles to form a shape. An attractor of this kind has no stable orbit and never repeats itself, yet no trajectory ever escapes a bounded region either. So a particle released anywhere gets drawn onto the same surface and then wanders it forever without retracing its steps. Chaotic and confined at the same time, which is exactly what made these things famous.
Particles start out in a small range and are scattered out to fit the attractor's much larger space on the first frame.