The possessive case of nouns is formed with an apostrophe: Keesha's costume, the wolf's fangs. But personal pronouns and the relative pronoun who change form to show possession.
My house is bigger than your house.
His anger evaporated in the face of her explanation.
The bulldog bared its teeth at us.
Our decision affected their plans.
The economist, whose book had received good reviews, agreed to speak.
Your plans are more definite than ours.
Be careful not to confuse possessive pronouns and nouns with contractions. Possessive pronouns do not have apostrophes. You have to distinguish between its/it's and whose/who's. The possessive of it is its, not it's; the possessive of who is whose, not who's. It's and who's are contractions ( it's = it + is; who's = who + is).
The cat lost its whiskers. ( NOT it's whiskers)
It's ( It is) Friday!
Whose backpack is this?
Who's ( Who is) the author of the book?