Here's why:
* Common nouns refer to general things or people, not specific ones. They are not capitalized unless they are the first word in a sentence.
* Proper nouns are specific names for people, places, or things. They are always capitalized.
For example:
* Common Noun: village (refers to any village)
* Proper Noun: Greenwich Village (refers to a specific village in New York City)
Let me know if you have other words you'd like to classify!