Here's why:
* Common nouns refer to general things or people. They are not capitalized.
* Proper nouns refer to specific people, places, or things. They are capitalized.
Examples:
* Common noun: pond, dog, city, teacher
* Proper noun: Lake Michigan, Fido, Chicago, Mrs. Smith
Since "pond" refers to any body of water that is smaller than a lake, it is a common noun.