The mechanism by which air enters the mammalian lung is difficult for many students of physiology. In particular, some students have trouble seeing how pressure can be transmitted through a fluid such as the intrapleural fluid and how the magnitude of that pressure can change. A Cartesian diver, an old-time child's toy, may be used as a visual aid during lecture to illustrate and make more understandable these hard-to-grasp concepts. The Cartesian diver is easy to construct from readily available materials. In addition to helping explain lung mechanics, the performance of the Cartesian diver takes most students completely by surprise and thereby serves to "wake up" the student whose mind may not be completely engaged in the topic. (Contains 2 figures.)