In this research, we conducted an elicitation study with 54 participants to evaluate user-defined mobile gestures given three types of operations, handheld postures, and interaction spaces, respectively, to investigate the effects of these factors on mental model. The results of the elicitation study revealed that each of the three factors significantly affected the users' mental models, which could be reflected by the characteristics (e.g., nature, number of steps, and spatial utility) of the user-defined mobile gestures. In addition, we found that users tended to assign different roles to mid-air and on-screen gestures based on their motor ability given a handheld gesture and their interpretation of the characteristics and requirements of an operation. We further derived a set of gestures commonly defined in the elicitation study for a list of representative operations, and we share our insights into users' mental models and the implications on future mobile-gesture design in different contexts. [ABSTRACT FROM AUTHOR]