In this episode it's the one about Hansel and Gretel children found dead by Joyce (Buffy's mum). There was a symbol on their hands and they had been investigating it.

Well, later on in the episode Willow says that it's for a prediction spell for buffy's birthday. She say's this when their lockers are being raided by Principal Snyder. Buffy then walks into the library to find Gile's books being taken away. She tells Giles what Willow had said. But instead of saying prediction, she says protection! You need to listen out very carefully for this one.