The Inside and Outside the Body activity helps students develop a conceptual understanding of anatomical barriers such as skin and mucus membranes that separate internal cells and fluids from the external environment. This short exercise prepares students for lessons in both anatomy and physiology and reflects recommendations from policy documents that suggest teaching core concepts. Understanding processes such as absorption, gradients and flow, and body defenses relies on the core concept of anatomical barriers. Instructors can use the concepts taught in this activity in subsequent discussions of topics such as immune tolerance of the fetus, the devastating impact of burns, and the sites of gas exchange.