Within-group similarities and between-group differences are used to illustrate the socio-cultural nature of the concept-building process in highly collaborative computer science classrooms. Simultaneously, a social constructivist perspective is used to describe the individual aspects of this development. The study uses written explanations from high school students as well as novice university students to illustrate the cognitive trajectory from initial hunches to a holistic knowledge of the concepts of keys in database modelling. The main findings of the study, however, are of a general epistemological nature, as they enlighten and exemplify the social processes of these classrooms as seen from a perspective of situated cognition. Based on these findings, the paper finally addresses implications for teachers. In particular, it is emphasised that teachers need to pay careful attention to their own use of language in discursive interaction with students.