Here's why:
* Common noun: Refers to a general category of things, not a specific person, place, or thing. "Stomach" refers to the general organ found in humans and many animals.
* Proper noun: Refers to a specific person, place, or thing. For example, "John" or "New York City."
* Abstract noun: Refers to an idea, quality, or state. For example, "love" or "happiness."
Let me know if you have any more questions!