Take a cardboard strip about one inch wide by ten inches long.
Gently bend the strip into an arch. Place the two ends on a table, and push down on the center of the arch. What happens? The strip goes flat on the table. Now, make the arch again, but put books against each end of the arch. Push down again at the center. What happens? The books keep the arch from flattening out. This is the thinking behind the construction of an arch bridge. It is one of the oldest designs in bridge building.
