This paper addresses the questions of what and how two mentoring primary teachers and two science primary student teachers learned from their common experiences while planning, implementing, and reflecting on different science teaching activities with students aged 7-9 during a four-week school practicum. During four weeks, two lessons each of the student teachers and two of the mentors were video recorded. In connection to each lesson, the student teacher and the mentor, working in pairs, reflected on the video recorded lesson in a stimulated recall session. The student teachers had had training in scientific knowledge but only brief experience of teaching. The mentors were well experienced in the pedagogy of primary teaching and mentoring, but did not feel confident with their science content knowledge. During the reflections the student teachers and the mentors expressed an increased understanding of both teaching and learning science, an improvement of their own practice, and a collaborative learning experience throughout the process of working, observing and reflecting together. The results support new insight into what learning comes from professional relationships built on the process of planning, teaching and reflecting together.