The apostrophe can be used to show who things belong to. If an item belongs to something, the apostrophe shows us who, by sitting at the end of the noun. If that noun doesn't end in s, the apostrophe ...

Oh, look! There’s some thing sleeping in the trees! Common nouns are the names of things, that’s people, places or objects, while a proper noun is the name of a particular person, place or thing.