We examined Finnish lower secondary students’ mental models of magnetism through their drawings, written explanations and interviews. Secondary students in Finland (N=12) engaged in six lessons designed specifically to target three key concepts in understanding magnetism: structure and organization (magnetic domains), magnetic fields and magnetic interactions. We describe how, with a finite number of key concepts introduced, students reflected upon and revised their mental models of magnetism and magnetic interactions towards more sophisticated and normative scientific views. We found two new categories of students’ models: the pole model and pole/field model. The critical moments in evolving the models happened during the investigations regarding understanding magnetic fields and magnetic internal structure. This article gives an example for teachers and researchers of how to follow students’ development of mental models in science.