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Image via WikipediaAir conditioning 

AC counts as a rare instance of innovation through sheer individual insight. After summer heat waves in 1900 and 1901, the owners of a printing company asked the heating-systems specialist Buffalo Forge Co. for a way to make the air in its press rooms less humid. The project fell to a 25-year-old electrical engineer named Willis Carrier, who built a system that cooled the air to a temperature that would produce 55% humidity. His idea ultimately rearranged the social and political map of America.