Here's why:
* Common Nouns: Refer to general categories of people, places, things, or ideas. They are not specific to any particular individual or place.
* Proper Nouns: Refer to specific people, places, or things. They are always capitalized.
Examples:
* Common Noun: plumber (refers to any person who works with plumbing)
* Proper Noun: Mario (refers to a specific person named Mario)
Since "plumber" describes a general occupation, it is a common noun.