Clarisse McClellan lives next door to Montag. Clarisse is innocent, curious, and full of life. She doesn't conform to the roles and regulations of the oppressive society that she and Montag live in. When she asks Montag if firemen used to put out fires rather than start them, Montag laughs. This is one of the first moments when Clarisse challenges Montag, challenges the way he thinks:
You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you.
Here, Clarisse quite literally tells Montag he doesn't think too deeply about things. Clarisse mentions that she doesn't watch the parlour walls and her family sits around and talks. This is strange to Montag whose wife is addicted to the parlour shows and has little to no experience of a family life full of conversation.
At the end of their first meeting, Clarisse asks Montag if he is happy. Again, Clarisse's curiosity causes Montag to think, even to evaluate his own life. Montag remarks that Clarisse's face was like a mirror:
How like a mirror, too, her face. Impossible; for how many people did you know that refracted your own light to you?
This image "reflects" the dynamic that Montag experienced with Clarisse. He has no meaningful conversation with anyone, let alone his wife. When he talks to Clarisse, she opens his mind and makes him think about things he's never considered and she makes him think about himself.