An eyedropper “diver”—eyedropper, dropper, diver, and Cartesian diver are used interchangeable in this activity—can be made to float, sink, or remain suspended in the middle of a water-filled, capped, plastic 2 L soda bottle “on command” when the bottle is secretly squeezed or released. In this activity, a standard, overly simplistic and incorrect textbook explanation is challenged. The Cartesian diver serves as a fairly transparent black box-nature of science system to explore how focused observations (empirical evidence) and critical questions can lead (via logical argument and skeptical review) to an explanation that is at odds with the standard (incorrect) textbook explanation.