This case study presents three true stories of people who suffered fatal hyponatremia (low sodium concentration in their body fluids) after consuming excessive amounts of water. Students examine the tonicity of the extracellular environment created by this water consumption, the effect this has on cells and how this impacts different tissues/organs in the body. After they understand that the immediate cause of death in these patients was due to brain swelling, students are then cast in the role of physicians who must confirm the diagnosis and administer emergency treatment. They also consider why certain populations (athletes, small children, and desert-dwellers) are at increased risk for hyponatremia. A final exercise asks students to calculate how much brain volume increases due to a specified change in solute concentration and a list of simplifying assumptions. The case was developed for an undergraduate introductory molecular and cellular biology class for majors, but is appropriate for any introductory biology, upper division physiology, or cell biology class in which osmosis is discussed.