[Theo has received a "D" and is explaining why he should not be punished]
Theo: You're a doctor and Mom's a lawyer, and you're both successful in everything and that's great! But maybe I was born to be a regular person and have a regular life. If you weren't a doctor, I wouldn't love you less, because you're my dad. So rather than feeling disappointed because I'm not like you, maybe you should accept who I am and love me anyway, because I'm your son.
Cliff: Theo... that's the dumbest thing I've ever heard in my life! No wonder you get D's in everything! You're afraid to try because you're afraid your brain is going to explode and it's going to ooze out of your ears. Now I'm telling you, you are going to try as hard as you can. And you're going to do it because I said so. I am your father. I brought you into this world, and I'll take you out!
[after Theo tells Cliff to love him for who he really is when failing his classes]
Cliff: That's got to be the dumbest thing I ever heard!
Vanessa Huxtable: Denise pushed us out of the bathroom and Rudy doesn't have all of the shampoo rinsed out of her hair. If she goes blind can we get a dog?
[knock on the bedroom door during a tender moment]
Cliff: Who is it?
[Vanessa and Rudy enter]
Cliff: No, no, no. When I say, "Who is it?", it doesn't mean for you to come in. When I say, "Who is it?", you say who it is.
[the girls retreat, close the door, and knock again]
Cliff: Who is it?
Vanessa Huxtable: [from beyond the door] Who it is!
Theo: [upon being woken up] Come on, Dad, five more minutes... Four? Three?
Cliff: Is this an auction?
Heathcliff Huxtable: And you're going to do it because I said so! I am your father!
Heathcliff Huxtable: Where are you going?
Denise Huxtable Kendall: I have a date, Daddy.
Heathcliff Huxtable: [referring to the belt on her leg] Uh, not like that, blood cannot get up to your brain from there! And besides, it's a school night.
Denise Huxtable Kendall: It's Friday, Daddy.
Heathcliff Huxtable: Yeah, but did you go to school today?
Denise Huxtable Kendall: Yeah...
Heathcliff Huxtable: And so it's a "school night".
Vanessa Huxtable: Dad, Denise pushed us out of the bathroom and wouldn't let me rinse the soap out of Rudy's eyes like Mom said and now Rudy might be blinded for life! If she is, can we get a dog?
Cliff: No, but I'll tell you what, you can *walk* your sister... down to my room and get her cleaned up!
Boyfriend in Pilot Episode: [points to his ear] Are you looking at my ear?
Cliff: No, I was looking at the earring in it.