We present excerpts from a research project initially focused on the inquiry and the development of prospective teachers' mathematical knowledge. The research started from the design of a particular task for prospective teachers built onto the request to interpret pupils' answers suitably selected. The focus of inquiry has gradually changed to include reflections on our own practice. Indeed by discussing with our students about the interpretation of pupils' answers, both our students and ourself have developed new mathematical knowledge and awareness. In this paper we propose a reflection upon this development, by conceiving our students and ourselves as co-learners.