To prepare to teach in mathematics classrooms where children learn to justify their thinking, the teachers themselves must have experienced an environment of inquiry, communication, collaboration, and reasoning. Teacher educators strive to establish such environments for preservice teachers (PSTs) and to support their transition from rules-based to reasoning-based curricula. This article examines the instructor's and students' roles in developing socio mathematical norms for PSTs to explain their thinking and justify mathematical ideas. It considers elements of one task that may generate deeper understanding of elementary school mathematics while building classroom norms for justification. Examining video and other records of teaching practice offers ways to document how PSTs collectively build on one another's ideas. (Contains 3 figures.)