Our work is in keeping with the general framework of didactical research in France, where we both studied, and now teach. This paper describes a model training session designed for in-service mathematics teachers, whose purpose is to guide them in their understanding of the relationship between students' tasks and activities in secondary school (from 11 to 18 years). We first explain the didactical tools that we consider necessary in order to address tasks analysis, and then, as an example of a training session, we provide a list of tasks on similar triangles illustrating the two kinds of analysis in which we engage participants. As a conclusion to this paper, we comment on our choices, and their potential influence on teachers' practices.