Giles: For as long as there have been vampires, there’s been the Slayer. One girl in all the world, a Chosen One.
Buffy: He loves doing this part.
Giles: Alright. The Slayer hunts vampires, Buffy is a Slayer, don’t tell anyone. Well, I think that’s all the vampire information you need.
Buffy the Vampire Slayer, Season 1: Episode 2 – “The Harvest”