Here's why:
* Common noun: A general name for a person, place, or thing.
* Proper noun: A specific name for a person, place, or thing.
"Stage" refers to a general concept, like a platform for performing or a phase in a process. It's not a specific, unique entity.
Examples:
* Common noun: The actors performed on the stage.
* Proper noun: The play premiered on Broadway. (Broadway is a specific location)
Let me know if you have any other words you want to categorize!