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.
Added:
Blair and her mother, Eleanor talk about Blair's father, which sets the younger Waldorf over the edge, leading to an intense bulimia episode.
Added:
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.
Added:
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.
Added:
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!
Added:
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!
Added:

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?