Computer input by scratching

This is the idea behind a scratch input — a system that would measure surface scratches and interpret them as commands. Researchers at Carnegie Mellon University are developing such an interface. In theory, all you'd need to turn any surface in to an input device would be a kind of stethoscope and a processor to interpret the scratches. The potential applications are innumerable: Scratch your computer to turn it on, or use your desk as a drawing surface for your fingernail, with the drawings appearing on the monitor.

Source: http://www.wired.com/gadgetlab/2009/08/scratch-input/