Method, device, and system of detecting a lie of a user who inputs data. A method includes monitoring input-unit gestures and interactions of a user that inputs data through an electronic device; and based on analysis of the input-unit gestures and interactions, determining that the user has inputted false data through the electronic device. A particular fillable field, or a particular question, are identified as having untrue input from the user. Optionally, spatial orientation data of the electronic device is taken into account in the determination process. Optionally, contextual analysis is utilized, to determine that the input-unit gestures and interactions reflect an attempt of the user to perform a beautifying modification of a data-item to his benefit.