It turns out a human body has a lot of surfaces facing the "outside" in some sense and we forget about the parts we can't see. Most of this surface is not cover

It turns out a human body has a lot of surfaces facing the "outside" in some sense and we forget about the parts we can't see. Most of this surface is not covered in what we'd conventionally consider skin. It's bit like if you were looking at surfaces in a house and forgot the walls and ceiling.