In France, since September 2016, new mathematics curricula ask primary schools teachers to integrate computer science, algorithmic, programming, using robots or new software like Scratch. Yet, the difficulty for ICT to penetrate mathematics classrooms is not new , explained in many research by the "teacher barrier". This presentation describes a case study from a research project “DALIE” (Didactics and learning of computer science in primary school), where 24 ordinary teachers (with no training), volunteered to use robots and/ or Scratch software. We focus here on the sessions of René, one of these primary school teachers, who uses for the first time Scratch. As most primary school teachers, René is a beginner in the functioning of this tool, but also in the knowledge that it embeds, and a fortiori in its didactic uses. How does Scratch become a teaching tool for René and for which aims? What knowledge and practices does he develop? What can be learned from this study in order to set up teacher training and to develop resources to support teachers? Based on the results of this study, we highlight here the importance of didactic “landmarks”, functioning as references in the dynamics involved along the development of teaching practices with ICT. The first part details the theoretical tools we use to analyse René's practices, based on our previous researches, and the second one the main results of our observations. The presentation will end by a discussion.