The purpose of this assessment probe is to elicit students’ ideas about solids. It is designed to reveal the macroscopic and/or microscopic properties students use to decide whether a material is a solid. 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 and arrangement of the atoms or molecules. High school students deepen their understanding of the behavior of solids, liquids, and gases based on the objects’ position, arrangement, and motion. This probe can also be used as a card-sort activity. Teacher notes, standards, assessment, and references are provided. This book selection also includes the Table of Contents, Dedication, Preface, Introduction, and Index.