Although decades of mathematics education reform supports using effective classroom discourse to increase students' learning of mathematics, research about what mathematics students learn in such classrooms is less developed. Moreover, how teachers actually facilitate classroom discourse and navigate through unpredictable terrain to develop students' understanding of mathematics remains challenging for teachers. In this paper, I examine the practice of an experienced middle school teacher as she leads her students in classroom discussions about perimeter of rectangles. Unpacking how she facilitates discourse in the classroom and addresses her students' mathematical ideas about perimeter shows how students' insights or misconceptions are identified and clarified, explained and illustrated, tested, and then affirmed or revised. Key findings of this case study include how the teacher uses a specific curriculum unit and divergent questions to support dialogic mathematical discourse, how she addresses a misconception, how she fosters a mathematical disciplinary community in the classroom, and how she copes with time constraints. Implications of these findings are discussed.