Clarisse’s family is different because they slow down.
Clarisse is a very unusual girl. She asks Montag if he is happy, and throws him completely off. She explains to Montag why her family is so unusual, because they do things so slowly in a society where everything is fast.
"Oh, just my mother and father and uncle sitting around, talking. It's like being a pedestrian, only rarer. My uncle was arrested another time-did I tell you?-for being a pedestrian. Oh, we're most peculiar." (Part I)
Clarisse explains that her family slows down and looks at things, like stretched-out billboards. They do not speed. They actually think about whether or not they are happy. She tells him that she is different from other girls her age because her family remembers a time when things were different.
But that was a long time ago when they had things different. They believed in responsibility, my uncle says. Do you know, I'm responsible. I was spanked when I needed it, years ago. And I do all the shopping and house-cleaning by hand. (Part I)
Clarisse says that her generation drives too fast and shoots at one another for fun. She does not consider it fun though. She does not consider watching too much television and running over each other entertainment. She also tells Montag that she is not afraid of him, and asks him if he reads the books he burns.
The exchanges with Clarisse really shake Montag, especially when he learns that she has died. He has never thought to question his life before he talks to her. He has never thought to ask himself if he was happy. Through Clarisse, the reader learns the dark side of Montag's pleasure-seeking society.