Buffy: When did you guys hang out?
Xander: Oh, she was fighting one of those, uh, apocalypse demon things, and I helped her. Gave her a ride home.
Buffy: And you guys talked?
Xander: Not extensively. No.
Buffy: Then why would you… Oh.
Willow: I don’t need to say ‘oh’. I got it before. They slept together.
Buffy the Vampire Slayer — Season 3: Episode 15 — “Consequences”