This multimedia lesson by the American Chemical Society explores how heating and cooling affect the motion of atoms in a solid. The inquiry-based lesson is supplemented by four interactive simulations that help students visualize particle configuration in solids and compare the molecular structure of solids and liquids. The classroom experiment uses a ball-and-ring apparatus to show expansion of a metal when heated. Included in the lesson is a student activity sheet with answer key and suggested discussion questions. Editor’s Note: The experiment may be done as a classroom lab or viewed as a video if the apparatus is unavailable to the teacher.