Hermione let his fantasy play out in his mind. It was a very Slytherin kind of fantasy, in a fashion. And she could guess why he enjoyed the fantasy.

"I doubt I could make an in Unbreakable Vow in regards to a sexual fantasy." She has seen how bad some Unbreakable Vows could get. And she is a lot more careful.

"But that doesn't mean that I wouldn't find a way to please you. To be obeying, but for you to know that the Lion is never quite tamed."