One of the first spells Amy casts is a “fake test paper”, when she makes her teacher believe that she has handed it over at the end of class. Xander (Buffy’s friend) witnesses this and later strikes up a conversation with Buffy and Willow, at which point we learn that Amy’s mum is a witch (according to Willow).
Xander then suspects that “it runs in the family” and is able to blackmail Amy over the fake test paper (i.e. to keep it a secret), so that she casts a “Love Spell” for him. This involves: “Diana Goddess of Love”, a necklace, several candles and (eventually) some swirling red smoke when the spell is cast. Almost immediately, the love spell goes wrong and Xander finds himself chased by every woman (instead of just the one he desired). As the spell also affects Amy, she gets “carried away” and turns Buffy into a rat (so that she can have Xander to herself). It is only with the help of Giles (Buffy’s watcher) that the love spell (and its consequences) are able to be reversed.