Physicists and physics students have been studied with respect to the variation in the ways they expound on their topic of research and a physics problem, respectively. A phenomenographic approach has been employed; six fourth-year physics students and 10 teacher-researcher physicists at various stages of their careers have been interviewed. Four qualitatively distinct ways of expounding on physics have been identified, constituting an outcome space where there is a successive shift towards coherent structure and multiple referent domains. The interviewed person is characterized as expressing an "object of knowledge," and the interviewer is characterized as a willing and active listener who is trying to make sense of it, constituting a "knowledge object" out of the ideas expressed and personal experience. Pedagogical situations of analogous character to the interviewer-interviewee discussions are considered in the light of the analysis, focusing on the affordances for learning offered by the different forms of exposition. (Contains 3 figures and 11 notes.)