Here's why:
* Common Nouns: These are general names for people, places, things, or ideas (e.g., cat, city, book, happiness). "Climb" doesn't name anything specific.
* Proper Nouns: These are specific names for people, places, or things (e.g., John, London, Eiffel Tower). "Climb" doesn't refer to a specific entity.
Example:
* Verb: "I will climb the mountain."
* Common Noun: "The climber reached the summit."
* Proper Noun: "The Mount Everest climb is challenging."