2 rules: 1. Writing Intention 2. What the noun ends with such as "s"
Singular:
That's the kid's toy.
That is the dog's bone.
Plural:
Those are the kids' toys.
Those are the dogs' bones.
Singular:
That's the kid's toy.
That is the dog's bone.
Plural:
Those are the kids' toys.
Those are the dogs' bones.