In this article, the authors share an episode from an after-school program and focus on eight of the twenty-four urban, sixth-grade students who volunteered to work on mathematical problem solving. When students are given a supportive environment, open-ended tasks, and thoughtful teacher questioning, their reasoning can develop naturally. By analyzing their reasoning as they explain and justify solutions, successful strategies that promote reasoning and justification can be learned. (Contains 6 figures and 1 table.)