Research on young children's reasoning show the complex relationships of knowledge, theories, and evidence in their decision-making and problem solving. Most of the research on children's reasoning skills has been done in individualized and formal research settings, not collective classroom environments where children often engage in learning and reasoning together to solve classroom problems. This study posits children's reasoning as a collective social activity that can occur in science classrooms. The study examined how children process their reasoning within the context of Grade 2/3 science classrooms and how the process of collectivity emerges from classroom interactions and dialogue between children as they attempt to solve their classroom problems. The study findings suggest that children's reasoning involves active evaluation of theories and evidence through collective problem solving, with consensus being developed through dialogical reasoning.