The purpose of this assessment probe is to elicit students’ ideas about conservation of matter. It is designed to reveal whether students recognize that the mass of a warm gas in a closed system is the same after it has been cooled, even though the volume it occupies has decreased. Elementary school students describe the properties of materials or objects and classify them as solids, liquids, or gases. Middle school students transition from focusing on the macroscopic properties of solids, liquids, and gases to explaining states of matter in terms of the position, arrangement, and motions of the atoms or molecules. High school students deepen their understanding of gases by learning about the gas laws. Teacher notes, standards, assessment, and references are provided. This book selection also includes the Table of Contents, Dedication, Preface, Introduction, and Index.