Here's why:
* Common nouns refer to general, non-specific people, places, things, or ideas.
* "Paintbrush" refers to a general type of tool used for applying paint, not a specific one.
Let's compare it to a proper noun:
* Proper noun: "The Mona Lisa was painted with a paintbrush." (Mona Lisa is a specific painting, so it's a proper noun.)
* Common noun: "I need to buy a new paintbrush." (This refers to any paintbrush, not a specific one.)