A method, a device, and a non-transitory storage medium having instructions to analyze a characteristic of an ultrasonic signal that propagated on a body of a user of the computational device and effected by an on-body action, performed by the user, in an area in which the ultrasonic signal has propagated; determine one or more sides of the computational device at which the on-body action is performed relative to the computational device; and select an input based on an analysis of the ultrasonic signal and the one or more sides.