I don't know why this man made his Apple Watch run a nearly 20-year-old OS, but I'm not going to question it. It's great.
YouTuber Nick Lee ported Mini vMac, an emulator for early Mac operating systems, to his Apple Watch after upgrading to watchOS 2.0. With the emulator he installed, he got his Watch to run OS 7.5.5, a version of System 7 which launched in 1996.
It's a little slow, and a little unwieldy since you have to use Watch's digital crown, but it works. There's definitely something to be said about an OS originally designed to work on a desktop computer running on something that can be worn on your wrist.
That's progress.