It is often claimed that people who smoke or are obese are more likely to get certain diseases (e.g., cancer, diabetes) and that this creates an external effect that raises the cost of medical care for everyone. Are smoking and obesity examples of behavior that create a negative externality? In thinking about your answer, consider that routine exercise and non-professional sports activities also result in numerous injuries each year, typically muscle and bone related injuries, which also raise the demand for health services and hence also the cost of medical care for everyone. Is exercising therefore a negative externality? Should exercise and non-professional sports activities be taxed to reflect the cost that it imposes on the medical care system, as is advocated for people who smoke or are obese?