The south pointing chariot was invented in China over 2000 years ago. I'm not sure that it was ever more than a curiosity. Still, it's a very curious and remarkable curiosity, and I can't imagine how anyone ever thought it was a possible project.
I don't think I could understand how it worked without having built one, but the video may be more comprehensible to other people.
Any other nominees for "how did that ever occur to them?" inventions?
Link found at Less Wrong.
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