In this paper we show how problem posing can be organized in the classroom. The decisions were made in a team of teachers and researchers: to use mathematical representations, to choose worked examples and to elaborate a tool called repertoire-instrument. Thus problem posing can be based on concrete representations, on the one hand, and on worked examples on the other. This case study focuses on a dialogue between a teacher and a student. The results concern the potentialities and effects of this device in the implementation in a class. The study is intended to be continued, with statistical analyses of the students' performance and qualitative analyses of the practices implemented.