One important challenge for researchers is to understand what happens inside classrooms that make the environment available for children's learning. This study develops the margin|centre dialectic as a general framework for describing participation of very young children in science-related activities. It employs a cultural-historical approach that goes beyond the classical theories and allows one to see learning as participation through a dialectical lens. The contradictions inherent in the concept of participation can be examined through the metaphor of the dialectical union of margin and centre, which, as with all dialectical units, embodies contradictions that constitute the drivers of change. This study analyses the events in a classroom of 4-year-old children in a public Brazilian kindergarten school focusing on the interactions that occur during a hands-on activity about an exploration of a physical-world phenomenon. The case study shows that (a) young children's participation is more than following teacher's instruction, (b) learning is a process of change in and through concrete social practice, and (c) viewed through the metaphor of margin and centre, participation and learning are constituted dialectically between individual and collective. (Contains 6 figures and 4 notes.)