Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things.
Since "bicycle" refers to any two-wheeled vehicle with pedals, it's not a specific name. It's a general term that could refer to many different bicycles.
Examples:
* Common noun: bicycle
* Proper noun: Schwinn (a brand of bicycle)