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.

Giles: 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”