1. A method, comprising: at a portable electronic device with a touch-sensitive display: displaying a plurality of icons on the touch-sensitive display, wherein a respective icon in the plurality of icons is assigned a respective charge, including a respective magnitude, in an electrostatic model in software; detecting a contact region when a user makes contact with the touch-sensitive display; determining that the contact region corresponds to one of the plurality of icons in accordance with the software-based electrostatic model; and activating a function corresponding to the one of the plurality of icons.