In this qualitative study we used a case study approach to observe and analyse a mathematics teacher who was challenged to redesign her lessons during network meetings with colleagues. Changes in practical knowledge are described by means of concept maps and semi-structured interviews. We applied cycles of change from the Interconnected Model of Professional Growth to describe the teacher's professional development. We show that the teacher's practical knowledge changes in the domain of practical content knowledge, and that controlled experimentation and reflection are necessary conditions to learn a new teaching strategy and to develop a series of lessons for pupils. (Contains 3 figures and 4 tables.)