This paper reports on a case study which explores the co-creation of a zone of proximal development (ZPD) in a mathematics teaching-learning activity in a Norwegian kindergarten. To capture the complexity of teaching and learning mathematics in kindergarten the study uses qualitative methods within an interpretative paradigm. The findings illustrate how a five-year-old girl and a kindergarten teacher co-create a ZPD by expanding each other's action possibilities, and how the co-creation is fundamentally based on mutual trust and responsibility. The results give insights into how mathematical learning possibilities may be promoted in kindergarten. The study illustrates the importance of being receptive to children's contributions and, above all, to trust children's abilities to take responsibility for moving mathematical teaching-learning activities forward.