Does covering the sides of a bridge make it more likely to sway in the wind? You might expect a solid wall to add strength. In crosswinds, though, a flat surface acts like a sail and catches more force. You build three model bridges from toothpicks and string. One has open sides. The second has sides half-covered with paper. The third has fully covered sides. You place each bridge one meter from an industrial fan and measure the tilt angle with a protractor. The open-sided bridge tilts only 3 degrees. The half-covered bridge tilts 18 degrees. The fully covered bridge tilts 37 degrees. More side coverage means more wind catches the bridge.