One aspect of scientific inquiry that appears to be particularly challenging to learn is how explanatory models are developed and used in science. It is even more challenging to learn to teach through methods that engage young students in building and using explanatory models. In part, this is because to do so requires that teachers make real-time instructional decisions in response to the ideas that students articulate. In this paper I present an example of a teacher who participated in a series of activities during a professional development course that guided her and her colleagues through the process of developing and revising an explanatory model of magnetic phenomenon. She subsequently transformed this series of activities to use with her elementary school students. Contrasting the series of activities in the elementary classroom to the corresponding professional development activities revealed improvisational instructional acts and shed light on aspects of the classroom context that necessitated and facilitated improvisation. In particular, this paper highlights the multiple pedagogical and disciplinary structures that teachers implicitly chose among when improvising. (Contains 2 tables, 4 figures, and 2 footnotes.)