Here's why:
* Common nouns refer to general categories of things, people, places, or ideas. They are not specific to one particular item.
* Proper nouns are specific names of people, places, or things and are always capitalized.
"Cereal" is a general term for breakfast food made from grains. It doesn't refer to a specific brand or type of cereal.
Examples of proper nouns related to cereal would be:
* Cheerios (a specific brand of cereal)
* Kellogg's (a specific cereal company)
* Frosted Flakes (a specific type of cereal)