Common nouns are words for people, places or things that aren’t specific (as opposed to a proper noun which refers to only one person, place or thing). Common nouns can be countable or uncountable, singular or plural.
There are cows are in the field.
We don’t know which cows those are, nor do we know the name of the field; these are common nouns because they can be replaced with any other cows and any other field, and the sentence would retain the same meaning.
I need some paper.
Would someone please answer the phone?
Does anyone have any ideas?
Where is that book I was reading?
Even though I’m looking for a specific book, I haven’t used the name of that book, i.e. the title, so the first letter should remain in lower case.
Where is Lord of the Flies?
The title is a proper noun, not a common noun.