Here's why:
* Common nouns refer to general categories of people, places, things, or ideas.
* Proper nouns are specific names for people, places, or things.
"Flour" is a general term for a powdered substance made from grains. It's not the name of a specific type of flour.
Examples:
* Common noun: Flour is used to make bread.
* Proper noun: King Arthur Flour is a popular brand.