Microsoft has innovated continually in developing novel interaction modalities, or natural user interfaces. Surface and Project Natal are two examples. While these modalities rely on sensors and devices situated in the environment, we believe there is a need for new modalities that enhance the mobile experience. We take advantage of sensing technologies that enable us to decode the signals generated by the body. We will demo muscle-computer interfaces, electromyography-based armbands that sense muscular activation directly to infer finger gestures on surfaces and in free space, and bio-acoustic interfaces, mechanical sensors on the body that enable us to turn the entire body into a tap-based input device.