This study investigates what teachers may learn by reflecting on lessons planned by others and by themselves. Our analysis assumes that teachers’ classroom practice is framed by the tasks proposed and the nature of the classroom communication established (Ponte et al., 2014). Central elements of teachers’ professional knowledge regarding classroom practice include knowledge of the mathematics to be taught, students’ learning processes and common difficulties, curriculum aims and connections, and instructional elements including tasks, materials, and ways of working in the classroom (Sherin, 2002; Shulman, 1987). We present the case of a grade 5 teacher, who taught a first lesson planned by the project team, based on the Decimat game, and a follow-up lesson planned by herself, aiming to address the students’ difficulties that she identified in the first lesson. We interviewed the teacher before each lesson to discuss the lesson plans and after each lesson to reflect on the classroom events. The first lesson unfolded very much as planned, except that it took much more time. The second lesson turned out to be quite different from the plan: the teacher presented students with a worksheet with four tasks and asked them to do the first two, which they did in 5 minutes. The remaining 40 minutes were spent on the discussion of these two tasks. The teacher decided to make this major departure from the plan as she identified new difficulties in the students concerning decimal representation. In teaching these two lessons the teacher became much more aware of how to conduct a discussion (Ponte et al., 2015) and the students’ difficulties with decimal numbers, but this awareness was stimulated by the detailed conversations that we had with her regarding the lesson plans and also the reflection on the classroom events (Schön, 1983).

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