"Full" is an adjective, not a noun.
Here's why:
* Adjectives describe nouns: "Full" describes the state of something (like a glass, a cup, or a room).
* Nouns are things: "Glass," "cup," and "room" are things.
Comparative forms of adjectives:
If you want to compare fullness, you'd use the comparative form of the adjective "full," which is "fuller."
Example:
* The full glass is heavier than the empty glass.
* The fuller glass is heavier than the half-full glass.