This article describes a specific approach to a teacher investigation of students' written work. The aim of the investigation was for in-service middle school teachers to externalize their thinking about their students' mathematical thinking and to then reflect upon these externalizations and make refinements accordingly. This aim mirrors the purposes of student-level mathematical activities entitled model-eliciting activities. Therefore, the teacher investigation was patterned after the design principles for student-level model-eliciting activities. These design principles and how the teacher investigation was modeled after these principles are described. In addition, the teachers' externalizations, reflections upon, and revisions to their thinking about their students' thinking are explained. The article concludes with general design principles that other teacher educators can use in designing their respective professional development contexts. (Contains 3 footnotes, 1 table and 1 figure.)