The purpose of this study was to describe how three primary teachers attempted to overcome incomplete content knowledge when teaching an astronomy unit. Daily observations of science activities were videotaped and transcribed from each classroom to determine the influences on the changes in teacher and student ideas of astronomy. Teachers' ideas were triggered toward the more scientific by classroom interactions. Influences on the experienced teachers' ideas were questions raised by the students, or conceptions students held of the content. Experienced teachers planned to elicit and address student ideas, and so were triggered to improve their understanding of the astronomy.