Paul is Sara's brother.
That's Carl's pencil.
It's Martin's school.
We use possessive 's with a person to talk about relatives and possessions.
- They are Lisa's parents.
- It's Jonh's car.
We use possessive 's with irregular plural people: men, women, children, people.
- These are my children's toys.
- Those women's skirts are too short.
We use possessive s' with regular plural people.
- They are my parents' friends.
We never use 's with things.
- The end of the story (NOT the story's end).