This paper addresses the developments of a class of fifth-grade children as they worked modeling problems during the first year of a 3-year longitudinal study. In contrast to usual classroom problems where students find a brief answer to a particular question, modeling activities involve students in authentic case studies that require them to create a system of relationships that is generalizable and reusable. The present study shows how 10-year-olds, who had not experienced modeling before, used their existing informal mathematical knowledge to generate new ideas and relationships, and how these developments were fuelled by significant social interactions within small group settings. (Author)