The concept of framing from anthropology and sociolinguistics is useful for understanding student reasoning. For example, a student may frame a learning activity as an opportunity for sensemaking or as an assignment to fill out a worksheet. The student's framing affects what she notices, what knowledge she accesses, and how she thinks to act. We find evidence of framing in easily observed features of students' collaborative behavior. We apply this observational methodology to explore dynamics among behavior, framing, and the conceptual substance of student reasoning in the context of collaborative active-learning activities in an introductory university physics course. We find evidence that certain student behaviors indicate and support a relatively sophisticated epistemological framing of these activities, one in which students discuss the substance of the ideas at hand. (Contains 1 table, 6 figures and 9 footnotes.)