Elizabeth isn't completely naive-- she knows when Ivan is flirting with her, even if she doesn't always understand what he's trying to suggest. And while she doesn't have much knowledge revolving around flirting and so on, she's read some books. Some very, very lightly-romanced books.

"DeWitts don't let their debts go unanswered-- I wouldn't have said anything if I hadn't meant it." She's stretching up too, her hands clasped around the thermos. Once she's close enough she puts her lips on Ivan's cheek, holds it for a moment, then lowers herself back down. Her face is considerably more pink.