International audienceThe aim of this paper is twofold: (1) to share didactic knowledge on the teaching of numerosity (especially in kindergarten) with teachers trainers and to awaken their "semiotic" vigilance on mathematical signs; (2) to use an interesting training modality, in that it favours exchanges and questions on professional knowledge, it allows to put accross some pedagogical tricks of classroom management, and confronts participants with the uncertainties and emotions of a subject in learning. The situation, of the homology-transposition type, was constructed for the training of specialised teachers for students with special needs. The didactic and semiotic analysis of the tasks allows to review mathematical and semiotic le...