Engineers often create small-size models of a new product to test its design. This is especially true with airplanes. Model testing tells engineers how a design responds to different air conditions and aircraft shapes, and lets them experiment with the control surfaces that are used to steer the aircraft. Using small models guides engineers to discard prototypes that do not work, which is a smarter option than than throwing away full-size (large and expensive to build) aircraft that do not work.