In this study, we examine how subjects set up, transform, and reason with models that they establish on the basis of known facts as they seek to explain a familiar everyday phenomenon--the phases of the moon. An interview schedule was designed to elicit subjects' reasoning, and in the case where explanations were mistaken, to induce a change in explanation. Detailed interviews of eight participants were videotaped and their reasoning analysed to highlight the difficulties encountered, the interaction between physical and geometrical aspects, simplification and idealisation processes, interplay between facts, concepts and visualisation, and the use of external visualisations through gestures and diagrams. We suggest that visualisation is an important process in science learning, and point to the importance of developing among students the ability to work with diagrams. (Contains 7 tables and 7 figures.)