Having grasped the concept, the Wrights remained puzzled as to how to attain it. During the summer of 1899, Wilbur stumbled onto the solution while working at the bicycle shop. While selling an inner tube to a customer, Wilbur removed the tube from its long, narrow box, and, as they chatted, he began to absentmindedly twist the ends of the box in opposite directions. As he did so, he suddenly visualized the box as a biplane, with the top wing connected to the bottom wing by the sides of the box. By twisting the corners of the wings in different directions-just as the pigeons did-he reasoned that lateral control could be maintained. That night, when he showed the box to Orville, his brother agreed, and they decided to use this "wing-warping" principle in their new glider, using a series of warping wires that ran through pulleys to the wingtips. A major problem in achieving controlled flight had been overcome.