Black teeth?

In the very first scene in which we meet Heath Ledger's love interest, she says in their youth, as a practical joke, Ledger's character put ink in her tea and it turned her teeth black for a month. She does the same later in the movie to get him back. Clearly, it's a way of flirting between the two of them.

She did it to him as a way to get back at him for him doing it to her years ago. When her's are also I took it as an obvious sign that they had been doing some pretty serious making out; which was a rather risqué thing for an unmarried couple of high society in those days.

Thank you! For years, I've wondered about that black teeth scene! I've always wondered why they drank that black stuff and was laughing about it. I thought it might have been liquid tar or something.