To make a singular noun possessive, add an apostrophe (') and an "s" to the end of the word such as "boy's bike". The possessive noun can also show aspects or attributes. Even If the singular noun ends in an "s", add an apostrophe and an "s" to make it possessive for such examples like "Kansas's population".
Plural nouns become possessive by adding an apostrophe. If a plural noun ends in an "s", just add an apostrophe to make it possessive such as "the Girl Scouts' badges". Do not confuse plurals with possessives. To form the possessise of irregular plural nouns that do not end in an "s" add an apostrophe and a "s" to make it possessive, such as "children's toys".
There are a few exceptions to the general rules. With names that end with an "eez" sound just add an apostrophe like "Achilles' heel". Use an apostrophe only to form the possessive for singular proper nouns that are formed from a plural word such as "United States' history". An exception is if the formation is of an abbreviation — then just add an apostrophe and an "s" as with "U.S.'s government". Never use an apostrophe when forming possessive pronouns like "hers".
If there is one noun and two possessors add an apostrophe and an "s" to the possessor nearest the noun, like "Joe and Mary's book". If there are two possessors who own separate things then both owners deserve an apostrophe and a "s" such as "Joe's and Mary's socks."