Modern mobile devices allow a rich set of multi-finger interactions that combine modes into a single fluid act. Such gestures may require the use of both hands: one holding the device while the other is interacting. While on the go, however, only one hand may be available to both hold the device and interact with it. In this demo, we present the Fat Thumb interaction technique, which uses the thumb's contact size as a form of simulated pressure. We present how this can be used, for example, to integrate panning and zooming into a single interaction. Contact size determines the mode (i.e., panning with a small size, zooming with a large one), while thumb movement performs the selected mode.