Orbital
In my computer lab job I often left a Mac running a screensaver that was a very simple physics simulation. Even on the computers of the day, it was a simple thing—not an impressive tech demo. But it could mesmerize me as I just watched it while using the phone.
I’ve tried many times to find that original screensaver, to no avail. I once recreated it faithfully (to my memory) in Processing, but lost that code. So when I wanted to learn PICO-8, it was an obvious exercise, and when I posted it on the BBS it was gratifying to find others also entranced.