When we want to express that something belongs to, or is associated with, something else, we use an apostrophe followed by the letter “s” to express that. For example:
The cat’s toy
Note that the nouns doing the possessing above are both singular. When we make the noun plural by adding “s,” or if the noun itself already ends in “s,” then the apostrophe goes after the “s.” For example:
All the cats’ toys (plural cats)
Several nations’ currencies (plural nations)
The mess’ origin (singular mess ends in “s”)
The brass’ patina (singular brass ends in “s”)
Do not use an apostrophe when you are only making a noun plural, not possessive; just add “s” (or “es” if the noun already ends in “s”):
I have three cousins.
They make messes.