Tapping your forearm or hand with a finger could soon be the way you interact with gadgets.
US researchers have found a way to work out where the tap touches and use that to control phones and music players.
Coupled with a tiny projector the system can use the skin as a surface on which to display menu choices, a number pad or a screen.
Early work suggests the system, called Skinput, can be learned with about 20 minutes of training.
"The human body is the ultimate input device," Chris Harrison, Skinput's creator, told BBC News.
To get around this Mr Harrison, a PhD student in computer science at Carnegie Mellon and colleagues Desney Tan and Dan Morris from Microsoft Research, use sensors on the arm to listen for input.
A tap with a finger on the skin scatters useful acoustic signals throughout the arm, he said. Some waves travel along the skin surface and others propagate through the body. Even better, he said, the physiology of the arm makes it straightforward to work out where the skin was touched.
Differences in bone density, arm mass as well as the "filtering" effects that occur when sound waves travel through soft tissue and joints make many of the locations on the arm distinct.
Software coupled with the sensors can be taught which sound means which location. Different functions, start, stop, louder, softer, can be bound to different locations. The system can even be used to pick up very subtle movements such as a pinch or muscle twitch. -Read more