The role of a teacher is different from that in traditional mathematics instruction when the implementation of a lesson is based on students' ideas. The author's experience teaching the same lesson (of the latter format) to two different classes of pre-service teachers in an elementary mathematics methods course is described. Since whole-class discussions were conducted using student presentations, the two classes "looked" very different, emphasizing the unpredictability of the teaching method. The practical example serves as a context to discuss the implications for the role of a teacher when preparing for and conducting lessons centered on students' approaches to problems. (Contains 10 figures.)