Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things and are always capitalized.
Examples:
* Common Noun: Movie (refers to the general category of films)
* Proper Noun: Avatar (refers to a specific movie)
So, while "movie" is a common noun, a specific movie title like "Avatar" would be a proper noun.