Research indicates that student understanding is either confirmed or reformed when given opportunities to share what they know. This study was conducted to answer the research question: Will classroom dialogue facilitate students' understanding of lunar concepts related to geometric spatial visualisation? Ninety-two middle school students engaged in classroom discussion about the two-dimensional models of the Sun-Earth-Moon system they created at the end of an inquiry Moon unit. Their responses to selected questions of the Lunar Phases Concept Inventory were also analysed. Findings indicate that classroom discourse provides the optimal setting for students to deeply ponder their long-held misconceptions and reconstruct their understanding regarding the cause of lunar phases. (Contains 6 tables and 6 figures.)