This is one of 25 assessment probes from the book,” Uncovering Student Ideas in Life Science, Volume 1: 25 New Formative Assessment Probes”, by Page Keeley. All assessment probes in this collection are aligned to a particular science concept and field-tested by several teachers in classes of diverse student backgrounds. The purpose of this assessment probe is to elicit students’ ideas about how the body is an interacting subsystem composed of cells. The probe is designed to see if students recognize that multicellular organisms are an organized group of cells and not a structure that is filled up with cells. The resource can be used to engage students in the topic at the beginning of a unit on the human body or to assess their understanding along the way.