Here's why:
* Proper nouns name specific people, places, or things (e.g., John, London, the Eiffel Tower).
* Common nouns are general names for people, places, or things (e.g., boy, city, building).
Since "rice" is a general term for a type of grain, it is a common noun.
If you were talking about a specific brand of rice, like "Uncle Ben's Rice," then that would be a proper noun because it names a specific product.