One of the most common household polyhedra is a cereal box. For over 100 years, American companies like Kellogg's™ have packaged cereal in containers shaped like rectangular prisms. Why is a rectangular prism the most commonly used solid for holding cereal? Would another design work equally or more efficient? This article describes an open-ended measurement activity in which students explore these and other questions. While the author used this activity for her ninth-grade geometry classes, it could be modified for lower grades.