A computer-implemented process for identifying which finger of a user's hand is touching a handheld mobile computing device (100; 200; 300; 400), comprising: using the handheld mobile computing device to perform the following process actions: detecting whether a part of a user's first hand is touching a signal injection site (102; 302) on the exterior of the handheld mobile computing device; if it is detected that said part of a user's first hand is touching the signal injection site, injecting an ultrasonic signal into that part of the user's first hand for a prescribed period of time; capturing a signal emanating from a finger of the user's first or second hand that is touching a signal sensing site (108; 308, 310, 312, 314) on the exterior of the handheld mobile computing device during at least the period of time said ultrasonic signal is injected into the user's first hand, said captured signal being caused by the injected ultrasonic signal traveling through the user's body to said finger; determining whether the captured signal matches, to a prescribed degree, a one of one or more signal sub-models included in a pre-established signal model, each of said signal sub-models being indicative of a signal expected to be captured from a different finger of the user's first or second hand whenever that finger is touching a signal sensing site during at least the period of time said ultrasonic signal is injected into the user's first hand; and whenever it is determined that the captured signal matches, to the prescribed degree, one of the one or more signal sub-models, designating the finger associated with that sub-model to be touching the handheld mobile computing device. using the handheld mobile computing device to perform the following process actions: detecting whether a part of a user's first hand is touching a signal injection site (102; 302) on the exterior of the handheld mobile computing device; if it is detected that said part of a user's first hand is touching the signal injection site, injecting an ultrasonic signal into that part of the user's first hand for a prescribed period of time; capturing a signal emanating from a finger of the user's first or second hand that is touching a signal sensing site (108; 308, 310, 312, 314) on the exterior of the handheld mobile computing device during at least the period of time said ultrasonic signal is injected into the user's first hand, said captured signal being caused by the injected ultrasonic signal traveling through the user's body to said finger; determining whether the captured signal matches, to a prescribed degree, a one of one or more signal sub-models included in a pre-established signal model, each of said signal sub-models being indicative of a signal expected to be captured from a different finger of the user's first or second hand whenever that finger is touching a signal sensing site during at least the period of time said ultrasonic signal is injected into the user's first hand; and whenever it is determined that the captured signal matches, to the prescribed degree, one of the one or more signal sub-models, designating the finger associated with that sub-model to be touching the handheld mobile computing device. detecting whether a part of a user's first hand is touching a signal injection site (102; 302) on the exterior of the handheld mobile computing device; if it is detected that said part of a user's first hand is touching the signal injection site, injecting an ultrasonic signal into that part of the user's first hand for a prescribed period of time; capturing a signal emanating from a finger of the user's first or second hand that is touching a signal sensing site (108; 308, 310, 312, 314) on the exterior of the handheld mobile computing device during at least the period of time said ultrasonic signal is injected into the user's first hand, said captured signal being caused by the injected ultrasonic signal traveling through the user's body to said finger; determining whether the captured signal matches, to a prescribed degree, a one of one or more signal sub-models included in a pre-established signal model, each of said signal sub-models being indicative of a signal expected to be captured from a different finger of the user's first or second hand whenever that finger is touching a signal sensing site during at least the period of time said ultrasonic signal is injected into the user's first hand; and whenever it is determined that the captured signal matches, to the prescribed degree, one of the one or more signal sub-models, designating the finger associated with that sub-model to be touching the handheld mobile computing device.