> 19. Given a solid ball in 3‑dimensional space, there exists a decomposition of the ball into a finite number of disjoint subsets, which can then be put back together in a different way to yield two identical copies of the original ball. While you're at it, you can completely turn that sphere inside out without creating any holes or creases[1]. [1] [this video]
Here are two classic youtube videos on the topic of sphere eversion; they mostly overlap, but I am posting both for double the hilarity in the comment sections :) (the full 21 minute one) [this video] (the first 10 minutes) [link] If I remember correctly, I think the 10 minute one has the more hilarious comment section.
Video list and Hacker News comment labels from HackerTube (prompted by @karpathy), released under ODC-By 1.0, built on the open-index/hacker-news archive. Hacker News data is provided by Y Combinator; comments remain the work of their authors. Video titles, channel names and thumbnails belong to their creators and are shown via YouTube. HackerTube Watch is an unofficial edition. Data export of 2026-10-31.
Plays via YouTube's privacy-enhanced embed
3 Hacker News comments
From the HN archive; up to 20 kept per video, favoring different commenters. Recommendations first.