As an instructor in courses for prospective teachers, I am interested in my students' ideas and ways in which they reconstruct their ideas in conversations about science. In the case study presented here, I reflect upon (a) ways in which I engaged prospective teachers in thinking together about their observations of a natural phenomenon, the changing phases of the moon, and (b) the ideas that they expressed. The case study examines ways in which prospective teachers developed understandings of the sun-earth-moon system from two perspectives: as viewed from the earth and from outside the solar system. (Contains 7 figures and 1 table.)