Blair is completely playing it off (please, she knows why Serena's asking her) when Serena asks about Chuck! Then Blair gets really defensive when Serena questions her judgment.
Blair and her mother, Eleanor talk about Blair's father, which sets the younger Waldorf over the edge, leading to an intense bulimia episode.
Serena is drunk last Thanksgiving. Dan saves Serena from being run over, and loves her so much, even if she doesn't know he exists.
The kids are talking about the Rufus-Lily dating revelation from earlier that Thanksgiving Day. Blair is equally surprised by the garage door in Dan's bedroom.
Nate brings a drunk Serena to get cleaned up in a flashback of the previous year. They get into a water fight with the shower head. Blair checks on them... and gets hosed down herself!
A sneak preview of "Blair Waldorf Must Pie," the ninth episode of the first season of Gossip Girl. A naive Dan, trying to be a smooth operator, invites Serena and family over for dinner!

Gossip Girl Season 1 Episode 9 Quotes

Serena: What's Chuck doing today?
Blair: Why are you asking me?
Serena: Well, you guys are friends. And, Blair, you know you can tell me anything. I'd be the last person to judge anyone.
Blair: With good reason.
Serena: Blair, I saw you with Chuck.
Blair: [pauses] I don't know what I was thinking. I mean, sleeping with him once, maybe I could understand. But twice?
Serena: Wait, you slept with him!?
Blair: Shhh.
Serena: Blair!

Serena: You and Nate get back together?
Blair: You mean, since Gossip Girl published pictures of him and some skank?