Inflatable Mouse is a volume-adjustable user interface. It can be inflated up to the volume of a familiar mouse, but be deflated and stored flat in a PC card slot of a laptop computer when not in use. Inflatable Mouse functions just like a typical mouse; moreover, it provides new interaction techniques by sensing the air pressure in the balloon of the mouse. It also addresses some issues associated with pressure-sensing interactions such as the lack of bi-directional input and the lack of effective feedback. Moreover, it can be used as both a control tool and a display tool. In this paper, the design of an Inflatable Mouse prototype is described and potential application scenarios such as zooming in/out and fast scrolling using pressure control are explained. We also discuss the potential use of Inflatable Mouse as an emotional communication tool.