This paper discusses how students' participation in conversation and classroom activities potentially evidences and constitutes their cognition. Participation is viewed in terms of reflective discourse, a construct from the literature, and is described in the context of two Year 11 students together designing a simple aplet for their graphics calculators, then discussing its operation. Reflective discourse is characterized by shifts in conversation so that concepts which are discussed initially as resulting from mathematical operations (calculations) become referred to, in turn, as objects that are operated on, to solve problems or for developing other concepts. The aplet was for calculating the magnitude of vectors given in component form. Interaction with each other, which centered on the technology, was seen to be instrumental to the students moving from understanding magnitude in its component definition, to later using magnitude to solve vector problems in an insightful way. Using reflective discourse as a framework for analysis suggested it is a valuable theoretical viewpoint for describing how learning might occur. (Contains 33 references and 10 figures.) (Author/ASK)