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The Screw.
One Good Turn: A History of the Screw
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Ohhh... good suggestion, but the nail & hammer is pretty good too.
I made the post originally because I thought about the line from Donnie Darko where they talk about sanitation and antibiotics being the greatest invention, and I felt that the lock and key was more ingenious. However, when I went to post I had a hard tiem not picking the light bulb, too many uses, television and the combuter wouldn't be up there without the light bulb.