The term "external surface" refers to the outermost layer of an object or structure. This surface is what is visible and often interacts with the environment. For example, the external surface of a building includes its walls, windows, and roof, which protect the interior from weather and other external factors. In biology, the external surface of an organism, such as the skin of a human or the shell of a turtle, serves important functions. It acts as a barrier against pathogens, helps regulate temperature, and can play a role in sensory perception. The characteristics of these surfaces can vary widely among different species.