Argumentation is credited with having a major role in mathematics learning and previous studies have shown that young children engage in mathematical argumentation, before entering primary education. In this paper a case study is presented which illustrates how preschool children collaboratively develop a mathematical argumentation chain in a learning situation with little guidance by a student teacher. The role of material-based action is highlighted for the development of an argumentation process amongst a group of preschool children. During the process, the data supporting the claim are gradually refined, as a reaction to continuing challenges from within the group, until a mathematically correct argument was established.