We use contractions all the time for informal writing. In forming contractions, the apostrophe is a "placeholder" for the missing letter or letters.

Some other contractions:

To make a singular noun possessive, simply add 's. For example:

The car belonging to Sue = Sue's car

The homework of Charles = Charles's homework

The entrance of the building = The building's entrance

The possessive form does not always mean ownership. It is also used to show that a person or thing has a particular quality or feature:

 Janet's happiness

The ocean's color

The plural form (meaning more than one) of most English nouns ends in s. Add only an apostrophe to make these words possessive.

The bicycles belonging to two boys = Two boys' bicycles

The contents of three boxes = Three boxes' contents

The discussion of those ladies = Those ladies' discussion

For any plural noun that does not end in s, add 's to show possession. For example. . .

 In class, we are discussing women's rights.

This library has very few children's books.

 NOTE: It's is always a contraction for it is. (It's sunny today.)

 Its is possessive. (The dog lost its bone.)