Two hundred and fifty years ago the Japanese Zen master Hakuin asked the question, "What is the Sound of the Single Hand?" This koan has long served as an aid to meditation but it also describes our new interaction techinique. We discovered that gentle fingertip gestures such as tapping, rubbing, and flicking make quiet sounds that travel by bone conduction throughout the hand. A small wristband-mounted contact microphone can reliably and inexpensively sense these sounds. We harnessed this "sound in the hand" phenomenon to build a wristband-mounted bio-acoustic fingertip gesture interface. The bio-acoustic interface recognizes some common gestures that state-of-the-art glove and image-processing techniques capture but in a smaller, mobile package.