This is the first appearance of Jeff Perry (Thatcher Grey) since "Testing 1-2-3".

Meredith: (voiceover) Before we were doctors, we were med students, which meant we spend a lot of time studying chemistry. Organic chemistry, biochemistry. We learned it all. But when you are talking about human chemistry, only one thing matters -- either you’ve got it or you don’t.

Derek: What's new with you?
Meredith: I saw my half-sister half naked.

Cristina: I am painting Burke's apartment so I can stop calling it Burke's apartment. A happy color. Red. Like blood.

Izzie: I love George. Oh, God I do, I love him, and I'm so happy because he's kind and smart and um, sweet and sexy. He's perfect, perfect, except for the fact that he kisses like a chicken. You know a chicken pecking the ground for food, justs pecking and pecking and pecking, and when he's pecking at me like that I forget that I love him, I forget that he's kind and sweet and sexy and I just wanna scream, "Stop pecking at me!" Did he peck you like a chicken, Mer?

Derek: Oh, I'm late. Dropped off the Chief's dry cleaning.
Mark: You're doing his laundry?
Derek: I'm helping out.
Mark: Yeah, what's he doing?
Derek: He has an extensive DVD collection.
Mark: You're dating the Chief. Huh.

George: Hey.
Meredith: You look weird.
George: I don't look weird.
Meredith: What's wrong?
George: I look fine.
Meredith: I know you.
George: It's horrible. The sex. With Izzie...(whispering) horrible. It's like she's trying to hard it's ... you ever seen a porno? Not that Izzie's a porno; she's an angel, but it's like she's trying to ... channel a porn star and she's trying to act all dirty and sexy, which sounds great, right? But in reality I just wanna say, "Izzie, just because you can do that with your legs doesn't mean that you should.
(Meredith cringes and sticks her fingers in her ears)
Meredith: Uh ... I wanna run.
George: Run, run. Run now!

Episode Title: Physical Attraction, Chemical Reaction

"Physical Attraction, Chemical Reaction" is the title of a song by Madonna.