The purpose of this assessment probe is to elicit students’ ideas about elements and compounds. It is designed to find out how students classify a common substance, water. The probe is best used with students in grades 5–12, preferably after they have encountered the terms element and compound. It can be modified to gather students’ ideas about whether water can be an atom, molecule, or both. The probe is provided in English and Spanish. A materials list, teacher notes, standards, assessment, and references are provided. This book selection also includes the Table of Contents, Preface, Acknowledgments, Dedication, About the Authors, Introduction, and Index.