Possessive or Plural -s?
Look at this sentence:
My father's car has 2 doors.
One -s is plural while the other is possessive. Plural means not one--many. Possessive means that something in the sentence belongs to that person (or thing).
In effect, the -s in doors means that there is more than one car. The -'s in father's means that something belongs to the father--in this case the car belongs to the father.