Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* "Biscuit" is a general term for a type of baked good. It doesn't refer to a specific biscuit.
Examples:
* Specific noun: "The biscuit I had for breakfast was delicious." (This refers to a particular biscuit.)
* Common noun: "I love to eat biscuits with my tea." (This refers to biscuits in general.)