Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized.
* Examples: John, London, Mount Everest, the Eiffel Tower
* Common nouns are general names for people, places, or things. They are not capitalized.
* Examples: man, city, mountain, tower
"Glass" is a common noun because it refers to a general type of material.
However, if you are referring to a specific glass, like a particular brand or a specific glass object, then it might be capitalized. For example:
* "I have a beautiful piece of Waterford crystal glass." (Waterford is a specific brand of glass)
* "The glass on the table is full of water." (This is a specific object)