You're stupid.

[She says this like it's a fact, then smiles again]

It's being playful with someone. Pretending like you're attracted to them. Or, well, I guess it's not really pretending all the time... [She looks thoughtful, because how do you explain flirting?] It's like being nice to someone, but because you want them to be attracted to you? I don't know how to explain it. It's a basic thing.