Roughly 60 lines of JavasScript to create 200 beautiful particles. Each particle has a random size color, opacity, and velocity. When the particles hit the edge of the screen, they bounce right back. The result is something more than a little hypnotic. I love restarting it over and over again to see what different colors I end up with.
Things to Try:
The demo is using sketch.js, which is available free on GitHub. You can easily change the number of balls by changing the particleCount variable, or the allowed sizes/colors by changing numbers in the random() functions found the this.radius or this.rgba variables. Speed/slow the particles by changing the vx or vy values.