Here's why:
* Common nouns refer to general things or people. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: nose (the part of the face that you smell with)
* Proper noun: Mount Everest (a specific mountain)
* Common noun: dog (any dog)
* Proper noun: Lassie (a specific dog)
Let me know if you have any other questions!