take a cardboard strip about one inch wide by ten inches long, and gently bend the strip so that it creates an arch.
place the two ends of the cardboard strip on a table, and push down on the center of the arch. what happens? the strip becomes flat on the table. now, make the arch again, but this time place several books against each end of the arch. push down again at the center of the arch. what happens? the books keep the arch from flattening out. this is the thinking behind the construction of an arch bridge, one of the oldest designs in bridge building. 