Teaching and learning are time-dependant processes. It can be hypothesised that the content coherence during a teaching sequence is an important factor of learning. In this perspective, it is of great interest to follow the occurrences of a notion and the development of its meanings in the classroom discourse, all along the various sessions of a sequence. The purpose of this article is to propose a methodology for reconstructing the story of a particular notion, from the video recording of the interactions between a teacher and students, with the use of a software dedicated to qualitative analysis. The case study which is the basis of this work involves two classes in Lebanon, at Grade 7, during a teaching sequence about electricity. The study also shows the respective contributions of the teacher and students to the discourse and knowledge progression in the sequence. (Contains 7 tables and 2 figures.)