This article argues that the practices of reasoning and sense making are critical for developing students' mathematical literacy. Seven mathematics teachers collaborated throughout a school year to discuss the ideas proposed in Reasoning and Sense Making (NCTM 2009) and put them into practice through action research. Conducting action research, with cycles of experimentation and reflection, helps teachers align their practice with their vision. Co-author Keazer was the facilitator of the teacher group, and co-author Menon was an active veteran teacher. This article offers examples from Mr. Menon's classroom and describes how he generated a culture of reasoning and sense making, drawing on data collected throughout the collaboration. Making any pedagogical changes is not reducible to prescriptive steps. By sharing examples and strategies, however, it is the hope that Mr. Menon's story will empower other teachers to experiment with similar actions. (A bibliography is included.)