Clarisse is run over by a car, possibly by joyriding kids.
Montag is a fireman who begins to question his life and the very essence of who he is when he meets a teenage girl from his neighborhood named Clarisse. She does not seem to see him as an authority figure or fear him. Instead, she is amiable and curious. She asks him the questions he never asked himself.
He felt she was walking in a circle about him, turning him end for end, shaking him quietly, and emptying his pockets, without once moving herself. (Part 1)
Clarisse is only in Montag’s life for a short time, but they walk together and talk. He even tells her he feels like he has known her for years. After a week, she’s gone. He realizes his new routine has been disturbed, and he enjoyed the time they talked.
Clarisse and her family are non-conformists. Mildred tells Montag that Clarisse was hit by a car.
"No. The same girl. McClellan. McClellan, Run over by a car. Four days ago. I'm not sure. But I think she's dead. The family moved out anyway. I don't know. But I think she's dead." (Part I)
In the end, we can’t be entirely sure if Mildred really knew what was going on because she doesn’t seem to care about anyone other than her families on the television shows. Her complete lack of empathy shows what her society expects of people, and is ironic because Clarisse felt differently and before she talked to him Montag never would have noticed her disappearance.
At a few times in the book we learn that young people joyride around in their cars intentionally trying to hit people so that they will feel anything at all. Clarisse does not approve, but it’s possible that same thing happened to her. When Montag is almost hit, he wonders if it is the same people who killed Clarisse.