Do all common nouns start with a capital letter?

No, common nouns do not start with a capital letter unless they are at the beginning of a sentence or part of a proper noun.

Here's the breakdown:

* Common Nouns: These are general names for people, places, or things (e.g., dog, city, book). They are not capitalized unless they are the first word of a sentence or part of a proper noun.

* Proper Nouns: These are specific names for people, places, or things (e.g., Fido, New York City, The Lord of the Rings). They are always capitalized.

Examples:

* Common Noun: The dog barked at the mailman.

* Proper Noun: Fido barked at the mailman.

* Common Noun: I live in a city.

* Proper Noun: I live in New York City.

Remember: Proper nouns are like individuals, and we give them special names (and capital letters!). Common nouns are like categories, and we use lower-case letters for them.