We report a study of three prospective secondary science teachers' development of theories-in-action as they worked together in a group to explore collisions using both physical manipulatives and a computer simulation (Interactive Physics). Analysis of their investigations using an existing theoretical framework indicates that, as the group moved from physical experiments to the computer simulation, their attention shifted from planning their experiments to processing system feedback, which impeded the iterative refinement of their theories-in-action. The nature of the theories they developed also changed. Learners' attitudes toward science and prior experiences affected the exploration process in both environments. In particular, prior instruction in physics and an authoritarian view of science seemed to impede engagement in the development and testing of theories-in-action. Certain features of the computer system itself also impeded exploration.