We investigated how prospective teachers used physics content knowledge when analyzing the talk of elementary children during special activities in an undergraduate physics content course designed for prospective teachers. We found that prospective teachers used content knowledge to reflect on their own learning and to identify students' science ideas and restate these ideas in scientific terms. Based on this research, we inferred that analyzing children's ideas through videos provides a meaningful context for applying conceptual physics knowledge in physics courses. Activities that are embedded within a disciplinary curriculum, such as those studied here, may help prospective teachers learn to use disciplinary knowledge in exactly the type of activity in which their content knowledge will be most useful: listening to and interpreting children's science ideas.