An Input Device You'll Always Have Handy

In addition to offering nearly fail-proof feedback, using body parts as input devices also has another distinct advantage: the device is literally always with you — because your body is you.
Yes, people often carry their mobile phones, but they'll never be without their hands or their ears. Thus they'll never be without system functions that have been assigned to their hands or ears.

While we're on the subject of novel user interfaces.