Functions form a central part of the U.S. mathematics curriculum especially at the high school level. There is a considerable body of research showing that students at all levels, including preservice secondary mathematics teachers, have difficulties with the definition of function as a correspondence between two sets with a univalence condition. Those difficulties include privileging algebraic representations and reductive interpretations of the univalence condition in the form of the vertical line test. In our research study, 47 pre-service mathematics teachers provided definitions of function, engaged with an interactive applet that had a non-standard representation of function, and then provided revised definitions of function. The results of the study show a measurable increase in the participants level of abstraction in their definitions, and an increase in their attention to the univalence condition. [For the complete proceedings, see ED606556.]