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Mary Babnik Brown's hair was particularly valuable to the military during World War II because it was used in scientific instruments to measure humidity, which was critical in the production of military aircraft and other equipment. Her hair, which was 34 inches long and untreated, was found to be sensitive to changes in temperature and humidity, making it ideal for these applications.

Specifically, her hair was used in devices like the Norden bombsight, where it served as crosshairs for bomb aiming. The military needed hair that had not been treated or dyed, as untreated blonde hair was deemed optimal for precision measurements in these critical wartime technologies (source: Wikipedia, Now I Know, Tampa Bay Times).

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Her story highlights the unique contributions of individuals to the war effort and the innovative uses of everyday materials in military technology.