This multimedia lesson by the American Chemical Society helps students understand that gases are matter with measurable mass, a topic of frequent misconception in the middle grades. Students compare the mass of a basketball before and after it is inflated, they explore the mass of a can of compressed air before and after air is expelled, and they consider how heating and cooling affect molecular motion in gases. Teachers have access to complete lesson plans for all the experiments, student activity guide sheets with answer keys, videos that depict how to do the demonstrations, and interactive animations for student use. This lesson is part of a larger collection of resources for middle school chemistry, covering matter, changes of state, density, the periodic table and bonding, the water molecule and dissolving, and chemical change.