The purpose of this assessment probe is to elicit students’ ideas about density. It is designed to find out whether students have commonly held ideas about mass, volume, shape, and other properties that interfere with their conceptual understanding of density. It is best used with students in grades 6–12, after they have had an opportunity to develop an initial concept of density. The probe may also be used with the card sort formative assessment classroom technique. 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.