The inscribed square/rectangle problem, solved using Möbius strips and Klein bottles.
Playlist with more neat proofs: https://www.youtube.com/playlist?list=PLZHQObOWTQDPSKntUcMArGheySM4gL7wS
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This argument was originally by Herbert Vaughan, appearing for examples in this issue of the Topology Proceedings.
https://topology.nipissingu.ca/tp/reprints/v06/tp06107.pdf
The on-screen argument for why all closed non-orientable surfaces must intersect themselves in 3d is a slight variation on one I heard from Dan Asimov.
2020 Paper by Greene and Lobb:
https://arxiv.org/pdf/2005.09193
Nice Quanta article about this result:
https://www.quantamagazine.org/new-geometric-perspective-cracks-old-problem-about-rectangles-20200625/
Timestamps:
0:00 - Inscribed squares
1:00 - Preface to the second edition
3:04 - The main surface
10:47 - The secret surface
16:45 - Klein bottles
22:38 - Why are squares harder?
25:10 - What is topology?
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