WWDD: S2E1 “WHEN SHE WAS BAD”
The Summers girls are tense in the ride home from LA. All summer, Buffy's been distant and mean, refusing to answer Dawn’s questions about being a slayer. The only thing Buffy's told her is that she's the slayer, she fights monsters, and Dawn can't tell anyone. Dawn was excited to learn Buffy's identity—her sister was a superhero—but since confronting her, Buffy's been more closed off than ever. Buffy’s managed to convince Dawn not to tell Joyce (for now) in exchange for Dawn's use of her hair and makeup supplies and permission to try on her clothes (hence why she had to do so much shopping).
School brings a welcome reprieve from the summer long tension. When Joyce drops Dawn off on their second day, her brand new, limited edition, Backstreet Boys (a new band taking the world by storm!) lunchbox breaks open and spills on Buffy’s pink camisole. They quickly drive home (which is what they are doing in their car scene) so Buffy can change into the white shirt she wears in the next scene. As a peace offering, Dawn washes out the stains so Buffy can wear it the next day of school (which she does!). When Buffy’s home, Dawn can hear her talking to Angel in her room. She momentarily worries that her sister might be in danger (because he’s a vampire) but she reminds herself that Buffy is a superhero and superheroes always save the day.
The next morning, Dawn tries to reach out to Buffy again, but Buffy snaps at her, telling Dawn to "stay out of it"--all she can do is get in the way. That night, Buffy knocks on Dawn’s door before coming inside and sitting on Dawn’s bed. Dawn can immediately tell something is different. Buffy thanks her for washing her camisole. She tries to explain she’s just trying to keep Dawn safe and the less she knows, the better. Dawn pretends to be asleep, angry at Buffy for not talking to her about her secret, and unable to understand how she could be endangered by having a superhero as a sister.