We report on how two middle-grade teachers supported their students' mathematical reasoning within the context of a novel modeling task in data analysis. We examine how the task features supported the development of teachers' knowledge as their students engaged with the task. Analyses of the teachers' practices suggest that the task features enabled teachers: (a) to develop new understandings of the mathematical content and the ways in which student ideas develop and are represented; (b) to adopt new roles in their interactions with the students, including a focus on listening and observing, and on asking questions for understanding and clarification; and (c) to engage in forms of interpretative listening that shifted the role of evaluation from the teacher to the student.