This paper points up, in the case of a particular class discussion, the crucial role that the Trace tool could play as potential semiotic mediator for the notion of function. In particular, the episode we are presenting here want to show how the idea of trajectory developed through a specific sequence of activities, carried out in Cabri and centered on the use of this tool can substantially contribute to building the meaning of function as a point by point correspondence. It also shows the conceptual difficulties attached to a complete construction of this meaning and how the role of the teacher is based on and complements the pragmatic experience of the students in Cabri. [For complete proceedings, see ED489632.]