Theory:

Capitalizing a common noun the rules are don’t do capitalization unless the common noun is the first word in a sentence or part of a title.  
Example:
State laws are passed by the legislature.
Each country may have varying laws.
Possessive common nouns show ownership or relation to something else. To make a singular common noun possessive, simply add an apostrophe and the letter “s”.
Example:
The baby’s cries woke the neighbours.
The toddler’s face was covered with papers.
The dog’s tail will never be straight.
To make the possessive form of a singular common noun that ends in “s” add an apostrophe and “s”.
Example:
The walrus’s bucket was full of fishes.
The bus’s green paint was looking odd.
For a plural common noun that doesn’t end in “s” simply follow the rules of a singular common noun that doesn’t end in “s” and add an apostrophe and “s”.
Example:
The men’s cricket team is headed for finals.
The children’s playroom was empty.
If a plural common noun does end in “s” just add an apostrophe to make the possessive form.
Example:
The play was interrupted by the babies’ crying in the back row.
The snakes hissings warned the snake charmer.
Reference:
www.k12reader.com/term/common-nouns