Can middle-grades students determine which of two rectangular prisms has a larger volume? Can they do so without using a formula? Geometry, and particularly the concept of volume, is important in many subjects, such as physics and chemistry. Students greatly enhance their mathematics knowledge when they make generalizations and construct arguments to justify their methods through problem solving. To realize this goal, teachers should encourage and accept students' self-initiative, allow students' responses to help shape lessons, and inquire about students' understanding of concepts before sharing their own. In this article, the authors describe a task that requires students to use their own nonformulaic methods and reasoning to investigate volume. (Contains 1 figure.)