system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where does the skin on the body stop being skin and become something else? Openings in particular. In the case of the face does it end at: lips? tongue? throat?

In the case of the rectum I assume the anal sphincter is the last skin hurrah before its all about intestines. Correct me if I'm wrong. What about the nose, the mouth, the ears.. etc.
gpt
Skin is a specific type of tissue- epithelial tissue. The other tissue types are connective tissue, nervous tissue, and muscle tissue. 

Epithelial tissue covers body surfaces and lines most of our internal cavities. Connective tissues function for support and protection, such as bone, fat, cartilage, blood, and lymph. Nervous tissue receive stimuli and conduct impulses (neurons). Lastly, muscle tissue makes up, well, our muscles- skeletal muscle, smooth muscle, and cardiac muscle.

When considering the rectum to the anus, the rectum is lined with epithelium and the anal sphincter is made up of both a smooth muscle component and a skeletal muscle component. There are tissue transitions at these junctions, so the tissue will slowly become less epithelial in nature while also taking on more qualities of the smooth muscle until it is eventually all smooth muscle and no longer epithelium.