The lesson study is a collaborative model of teacher professional development originating in Japan, which has received growing international attention. This multifaceted model can be considered from many points of view. This research aims to understand what physics and chemistry pre-service teachers learn, in the domain of the pedagogical content knowledge on the speed of sound, in a lesson study. Participants were all pre-service teachers (three) attending the first year of their initial teaching education programme. The lesson study, had two cycles, took place over the course of twelve sessions and a total of 36 hours of work. The first cycle consisted of ten sessions: the first eight sessions aimed at defining the topic and lesson planning; this was followed by the first research lesson taught by the cooperating teacher; and finally, there was a post-lesson reflection. The second cycle consisted of two sessions; a second research lesson was also taught by the cooperating teacher; and subsequent to this, there was a post-lesson reflection. This is a qualitative and interpretative study. Data were collected from participant observation of all sessions using field notes and video recording, individual interviews and individual written reflections. Results showed that the pre-service teachers developed the pedagogical teacher knowledge (PCK) to teach the topic in the planning phase, when they identified the students’ prior knowledge, which would help them learn the topic, and in the post-lesson reflection sessions, when they discussed strategies that could help students overcome their learning difficulties.