Soldiers are famously ordered to break step when crossing a footbridge, and the reason is resonance. Every structure has natural frequencies at which it prefers to vibrate; if a rhythmic force — marching feet, gusting wind, even synchronised pedestrians — happens to match one of them, each push arrives at exactly the right moment to amplify the last, and small inputs build into large oscillations. Modern engineers cannot remove a bridge's natural frequencies, so they do the next best thing: they add dampers that soak up the energy, ensuring the amplification never outruns the losses.