THE CONCEPT OF EPITHELIAL TISSUE:
Generally, epithelial tissues cover and line body surfaces, so you can think of epithelium as marking off our "insides" from our "outsides".
The skin is a type of epithelial tissue, the lining of the mouth is a type, and the lining of the stomach is another type.
If you placed a plastic shirt button on the skin of your arm, it would be considered "outside" of your body. If you cut through the skin of your arm and an underlying blood vessel, and forced that button inside the blood vessel, then the button would be considered "inside" your body.
Imagine that you put the same button in your mouth. That button would still be considered outside of your body because it has not passed across the epithelial lining of your mouth. Now, imagine that you swallow the button: It goes down your throat and continues into the stomach. The button rests in your stomach but does not pass into the bloodstream. That button is still considered to be outside the body. Since it is plastic, your body will not digest the button, so the button will continue through your digestive tract. Eventually it will leave your body through the anus. Since the button never crossed the epithelial lining of the tract that extends from your mouth to your anus, it was never considered to be inside your body.