Here's why:

* Mucous membranes are epithelial tissues that line cavities and organs that open to the exterior of the body. They are characterized by:

* Epithelial cells: These cells form a barrier and protect the underlying tissues.

* Connective tissue: This provides support and contains blood vessels.

* Mucus secretion: Goblet cells within the epithelium produce mucus, which lubricates the surface and helps protect against pathogens.

The mouth, along with the respiratory, digestive, urinary, and reproductive tracts, are all lined with mucous membranes.