Imagine a sensor that creates a 3D picture of its surroundings without invading on your privacy, that can look below the surface as well as detect minute movements. Walabot can do all that — in a sleek mobile phone-sized form factor. Even better, it can attach to a mobile phone and act as a portable scanning or monitoring device. Walabot uses radio waves to query its surroundings, be it through air or a material such as a plaster board. Here we demonstrate the capabilities of this novel sensor through two “handwaving user interface” applications: an “air piano” and a “page flipper”.