Here's why:
* Common nouns refer to general categories of people, places, or things. "Car" refers to any vehicle of a certain type.
* Proper nouns are specific names for individual people, places, or things. They are always capitalized. Examples include "Ford Mustang," "Toyota Corolla," or "Tesla Model S" - these are specific car models.
Let me know if you have any other questions about proper nouns!