While twisting a small pasteboard box with opposite ends removed, Wilbur observed that though the vertical sides were rigid endwise, the top and bottom sides could be twisted to have different angles at the opposite ends. Here was a simple means of warping the wings as they intended. They decided that a biplane’s wings could be twisted or warped in like manner, enabling them while flying in a glider to warp the wings on the right and left sides to present their surfaces to the air at different angles. By warping the wingtips the operator would be able to increase the angle of attack on one wingtip and decrease it on the other. Thus, they believed, the operator could obtain a greater lift on whichever side he needed it and less lift on the other side in order to assure lateral equilibrium. (They later had to modify this by adding a movable vertical tail.) To test their principle safely, the brothers built a model glider—actually a kite—with a 5-foot wingspan. Flown as a kite at Dayton, the model glider’s wing surfaces were warped by the use of four cords reaching from the upper and lower wingtips on each side to the operator on the ground. Balance from front to rear was maintained in part by an elevator tested variously at the front and rear, as well as by other means. The Wrights believed after the tests that the model glider had demonstrated the efficiency of their system of obtaining both lateral and longitudinal control. [Illustration: The brothers decided to test their glider at Kitty Hawk after a study of Weather Bureau records and the receipt of this letter.] _Why Kitty Hawk?_ Wilbur and Orville now proposed to build a full-size, man-carrying glider on which to test their method of control. Highly enthusiastic with the idea of gliding as a sport, they started thinking of a place for testing it. To get practice in operating the glider, they would first fly it as a kite before making gliding flights. For kite flying, steady winds and flat, open country were needed; for the gliding, a sandy area for soft landings and sandhill slopes free of trees and shrubs for low-level flights. The Wrights’ hometown of Dayton and its environs were not suitable for extensive glider trials. But because of their business demands, they wanted a site fairly close at hand. On May 13, 1900, Wilbur wrote his first letter to Chanute asking advice on a suitable location to test a glider. In this letter to the man who became their friend, mentor, and most important correspondent, Wilbur stated: