This article reports on a study of physics teachers engaged in collaborative action research. The purpose of the study was to examine and identify ways that teachers' knowledge about teaching and their educational situations grow when they are engaged collaboratively with other teachers in inquiry on their own practice. Individualist and social constructivist perspective were used to design the study and to collect and analyze data. Data sources included interviews of the teachers, classroom observations, transcripts of the collaborative action research meetings, and teachers' writing. The guiding methodology was naturalistic inquiry, and data were analyzed through the development of grounded theory to construct coding categories. A case study was written using the analysis. It was found that three mechanisms were used by the teachers to generate and share knowledge and understanding about their practices. This enhancement of normal practice includes anecdote telling, the trying out of ideas, and systematic inquiry. A second finding was that teachers' knowledge and understanding can grow through authentic being-in-the-world and through enhanced normal practice. The study suggests that if each of the mechanisms of enhanced normal practice is seen to be a legitimate form of research, then action research can be embedded in teachers' practice, and can then play an important role in teacher education and the reform of science education.