Your tablet computer and smartphone almost certainly use touch interaction; the capacitive sensing capability built into the display can distinguish particular kinds of touches. Disney's Touché system is a research prototype that can detect a variety of touches on different physical objects, including your body.
(Disney's Touché system video)
Using multiple frequencies allows the system to distinguish between a single finger, multiple fingers, a full-hand grasp and many other touch gestures. All you need is a single sensing electrode attached to the object at one end and a computer at the other, which analyses the changing signals to identify the particularly gesture in use.
Take a look at the 2:40 mark in the video shown above; it appears that you could create a water surface that could be used as a touch display, like Battlestar Galactica's Cylon datastream 'terminal.'
You could use the AMOEBA Tank display system to create the keys on the surface of the water; thanks to Moira for pointing out the datastream terminal in a past article.
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