This research looks at argumentation in the whole-class sharing portion of a second grade problem-centered classroom learning environment in which making sense of mathematical ideas was the accepted norm. This research analyzes nine episodes that identify various aspects, or characteristics, of argumentation. Videotapes, field notes, and interviews were used to observe and analyze student interactions and associated learning proclivities in the classroom episodes. The students openly shared their strategies and solutions for solving mathematical tasks. The teacher as argumentation facilitator was interviewed after each observation period. From my observations of the whole-class sharing time, argumentation emerged as the major ingredient in augmenting the students' process of mathematical sense making. The argumentation, which was inherent within this learning environment, provided the catalyst that helped students to effectively engage in, and to clarify and refine, their own mathematical thinking.