In science education, it is well understood that teachers' scientific conceptions can influence students' learning, but the extent to which students' knowledge influences teachers' ideas is not known. This paper describes a study of the mutual interaction of ideas between two second grade teachers, one teacher intern, and their classrooms during the course of an astronomy unit. In this case study, the teacher with the most content knowledge and the greatest ease in addressing student ideas was most influenced by the students. (Contains 16 references.) (WRM)