In this article we report how an approach was promoted from the pre-formal to the formal stage regarding the students' way of justifying mathematical reasoning. We implemented an activity based on the ACODESA teaching method to create a sociocultural environment in the mathematics class and to promote social interactions between the students. We present the case of a learner who changed his way of justifying after exchanging ideas and discussing with peers during the analysis of the justifications generated by each of them.