An old man in Japan owns a vintage topological puzzle, a little monster so difficult to solve that it ended up on Knight Scoop being taken to a physics professor, a professional magician and a chimpanzee in an effort to figure it out. Eventually a ring puzzle master shows how it’s done (10:40 if you’re pressed for time).
UPDATE: This is clearly the same puzzle Rusty blogged back in 2017 in a post that’s lost its YouTube. It’s now so famous there’s a website about the puzzle which sells replicas of it on Amazon.