In 1831, a column of soldiers marched across the Broughton Suspension Bridge near Manchester and the bridge collapsed. The cadence of the march happened to match a frequency the structure responded to strongly, and each footfall added a little more energy than the bridge could shed. Armies have broken step on bridges ever since. The physics behind that order is the same physics that makes a wine glass shatter to a held note and a car mirror blur at one specific engine speed: every structure has