Inspired by  

helen_c, I'm posting a question to all you native English, concerning English grammar.

Could someone please explain this to me: when something belongs to someone we write "yannik's cup", if it's plural, we write "girls' dresses", but what if someone's name, for example, ends with "s"? "Cornelius' hand", or "Cornelius's hand"?

A good way to learn the language, eh?