Devices, systems, and methods are disclosed for extending the functionality of a mobile device by including a cover for the mobile device with a substantial portion of the exterior overlaid with a touch-sensitive surface. Logic stored on the cover senses a user's touch on the touch-sensitive surface and transmits the touch input to the coupled mobile device. Logic stored on the coupled mobile device may receive and interpret the touch input as a function or command of an operating system, or a function or command of a program within the operating system of the mobile device.