Here's why:
* Proper nouns are specific names for people, places, or things (like "John", "New York", or "Amazon").
* Common nouns are general names for people, places, or things (like "boy", "city", or "river").
"Pizza" is a general name for a type of food. It's not a specific name for a particular pizza, like "Margherita Pizza" or "Pepperoni Pizza".
So, "pizza" is a common noun.