This study has investigated the use of an open guided inquiry laboratory course in which a group of pre-service teachers planned and implemented practical work for school purposes. A total of 32 pre-service teachers (physics, mathematics, and chemistry majors) participated in the study. Each participant wrote a reflective essay after completing the course, and three pre-service teachers were interviewed four times during the course. The results show that the use of an open guided inquiry environment provides support for pre-service teachers to discover the limits of their understanding of subject matter knowledge, allowing them to construct knowledge in a different kind of environment from any they had possessed previously, and helping them to understand the possibilities of practical work in teaching. In the course of developing their competence in these aspects, pre-service teachers also gain an understanding of various aspects of teachers' knowledge.