This article presents a descriptive account of an after-school study. The participants--Allan (male, ten years), Veronique (female, nine years), and William (male, ten years)--all spoke English as their first language and were considered by their classroom teachers to be meeting the provincially set expectations for the fifth-grade curriculum standards in mathematics. Analyzing the three fifth graders' spatial, arithmetic, and algebraic reasoning as they worked on a three-dimensional task revealed a contrasting perspective to the suggestion that their physical actions and interactions come before their sophisticated reasoning. As one of the five Process Standards, reasoning (and proof) is an essential part of acquiring and using mathematics. More than a means by which to confirm whether a solution is correct, mathematical reasoning involves exploring the mathematics at hand; generating, implementing, and evaluating conjectures; as well as justifying thinking and actions as one engages in mathematics. This article concludes with four important considerations for teachers as they nurture their students' mathematical reasoning. (Contains 8 figures.)