By the 1890s, the Wrights were interested in flight, especially the gliders of Otto Lilienthal. They started working on making airplanes in their bicycle shop. They thought controlling a plane was one of the big problems of flight. Lilienthal and others had been killed when they could not control their aircraft. The Wright brothers fixed the problem by inventing control surfaces, such as a rudder that would work in the air. They built wings that could be twisted a little and moved up and down slightly, to control flight. They called this steering system "wing-warping".