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Silk and soap settle a century-old flap

For more than 120 years, the English physicist Lord Rayleigh has had the last word on why flags flap in the breeze. A keen observer who figured out how the scattering of light makes the sky blue, Rayleigh attributed flag flutter to the interplay between deformations of a flag's surface and subtle gusting of the wind. Because the tiniest ripples in the flag and puffs of wind end up amplifying each other, even a smooth, or so-called laminar, wind inevitably causes the larger motions of flags, he reasoned.

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