Think aloud interview protocols from three high school post-physics students who interacted with a relative motion computer simulation presented in a predict-observe-explain format are analysed. Evidence is presented for: qualitative and quantitative difficulties with apparently simple one-dimensional relative motion problems; students' spontaneous visualization of relative motion problems; the visualizations facilitating solution of these problems; and students' memory of the on-line simulation used as a framework for visualization of post-test problems solved off-line. Instances of successful and unsuccessful mapping of remembered simulation features onto target problems are presented. Evidence from hand motions and other indicators suggesting that the subjects were using dynamic imagery in mental simulations during the treatment and post-test is presented. On the basis of these observations, it is hypothesized that for successful students, dissonance between their incorrect predictions and simulations displayed by the computer initiated the construction of new ways of thinking about relative motion, and that the memory of certain simulations acted as an analogue 'framework for visualization' of target problems solved off-line after the intervention. In such cases we find that interaction with a computer simulation on-line can facilitate a student's appropriate mental simulations off-line in related target problems. Implications for design and use of educational computer simulations are discussed.