* Common nouns refer to general, non-specific things. They are not capitalized.
* "Shoe boxes" refers to any box used to store shoes, not a specific box.
Let's contrast with a proper noun:
* Proper nouns refer to specific people, places, or things. They are capitalized.
* An example of a proper noun would be "Nike Shoe Box" if it referred to a specific type of shoe box made by Nike.