Decreasing student interest and achievement during the transition from elementary to secondary school is an international problem, especially in science education. The question of what factors influence this decline has been a widely discussed topic. This study focuses on investigating the relationship of elementary school teachers' content knowledge (CK) in physics upon the student outcomes of interest and achievement. Data were collected from K-4 elementary school teachers (N = 58) and their students (N = 1,326). Besides questionnaire surveys of teachers and students, one science lesson on the topic "states of matter and phase transitions" of each classroom was videotaped for assessing teaching quality. Analyses from a triangulation of data could not identify an impact of teachers' CK upon students' interest. However, the sequencing of learning processes within a lesson was found to be a positive predictor for students' achievement, although only minimal time was spent on reflective phases during the lessons.