What punctuation mark does a possesive noun always have?

Here's how it works:

* Singular nouns: Add an apostrophe and an *s* ('s). Example: *the dog's bone*

* Plural nouns ending in *s*: Add just an apostrophe. Example: *the dogs' bones*

* Plural nouns not ending in *s*: Add an apostrophe and an *s*. Example: *the children's toys*