Abstract
This paper examines how a first-grade geometry lesson became a site for exploring shape, selfhood, and belonging in an early childhood classroom. Drawing on interaction analysis of a lesson built around the picture book
The Greedy Triangle
, we show how children named personal strengths, identified shapes, followed a triangle through changing categories, reflected on why they liked being themselves, and used their bodies to form polygons. We ask what counts as mathematical activity in this lesson and argue that mathematics is not limited to naming shapes. It also involves noticing attributes, negotiating criteria, tracking transformation, coordinating bodies, and reasoning about recognition. We develop categorical belonging as an analytic construct for describing how children explore what something is, how it belongs to a category, and how it remains recognizable amid change. The analysis shows that everyday classification practices can become mathematical, embodied, and relational opportunities for children to negotiate recognition, difference, transformation, and belonging.