Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples include:
* People: John, Mary, Queen Elizabeth
* Places: London, Mount Everest, Pacific Ocean
* Things: iPhone, Statue of Liberty
* Common nouns are general names of people, places, or things. They are not capitalized unless they start a sentence. Examples include:
* People: man, woman, child
* Places: city, mountain, ocean
* Things: tool, statue, building
"Hammer" is a general name for a tool. It doesn't refer to a specific hammer.