Here's why:
* Common Noun: Refers to a general person or thing, not a specific one. "Lassie" is a general term for a young girl.
* Proper Noun: Refers to a specific person, place, or thing. "Lassie" as a specific character in the books and films would be considered a proper noun.
So, when you use "lassie" to refer to any young girl, it's a common noun. But when you're referring to the famous Collie dog character, it's a proper noun.