This study analyses teachers’ discernment of children’s mathematical knowledge development during preschool play activities and defines how children’s pre-knowledge supports or limits their learning possibilities. In total, two preschool teachers from different preschools participated in this research study. The data consists of two documentations of children’s activities, written notes, and reflections from these two participated preschool teachers. The teachers chose the samples and defined them as important examples of how to merge mathematical knowledge development in free play. Using variation theory, the focus was to define what aspects of children’s mathematical knowledge development were offered and how teachers discern children’s pre-knowledge. The results show how the preschool teachers discern children’s pre-mathematical knowledge in relation to development of abstract abilities, and the importance of identifying their participation in free play situations for promoting mathematical understanding.