Here are some main ideas from Part Two—"The Sieve and the Sand"—in Ray Bradbury's Fahrenheit 451.
First, Mildred is completely lost without the parlor walls (the TV screens) that she watches constantly. When they read in the hall, Mildred is distracted:
The parlor was dead and Mildred kept peering in at it with a blank expression as Montag paced the floor and came back and squatted down and read a page as many as ten times, aloud.
Montag seems to have been affected by Clarisse in the short time they knew each other. He could not remember the last time he had looked for the man in the moon, had not remembered there was dew on the grass in the morning, and felt funny opening his mouth to let the raindrops fall on his tongue. Here, he is very aware of nature:
Montag sat listening to the rain.
Montag reads something that analyzes when "friendship is formed." He thinks of Clarisse and wonders if that was what he had with her. It seems pretty clear that Montag really has no friends at all, not even Mildred, so he is curious as to relationship he had with Clarisse. The young woman made a deep impression on Montag. He thinks of her with the first book that speaks of friendship.
He picks up another book, entitled 'That favourite subject, Myself.' Mildred feels some kinship with this title as it immediately captures her imagination, but Montag again thinks of Clarisse:
But Clarisse's favorite subject wasn't herself.
Mildred is disgusted with the books. They aren't people, she notes, and she is obviously lonely—but not for Montag's company. We can assume that with the parlor walls, she and her friends (who are also obsessed) don't have to work at friendships because they are so passive, constantly entertained by characters that reflect the ideals of society: no original thought is required.
The concept of family is discussed. Montag told Clarisse in Part One that Mildred did not want children. The parlor walls give her everything she needs. Mildred says:
My 'family' is people.
She fears that if the house is burned down because they have books, she will lose her "family." She has no desire to read anything, ever. In truth, as Montag tries to convince her otherwise, Mildred becomes symbolic of society: neither will change.
Montag has (because of Clarisse) begun to recognize things around him and to question their presence. He studies the planes in the sky overhead.
How in hell did those bombers get up there every single second of our lives! Why doesn't someone want to talk about it?
Guy's conscience has been awakened; he cares for others. He has heard rumors that they are well fed, but that other countries are starving. He doesn't understand how they can "play" while others are going without food. He wonders how the world he knows has become what it is.
Montag is a very different man in Part Two than he was in Part One. Whereas he enjoyed being a fireman and living (he now knows) in ignorance, he now feels he cannot exist in the vacuum in which he has enjoyed his life so far. He has begun to learn and question. There is no going back for Montag. On the other hand, Mildred fights changed every step of the way. It is here where we see the rift that has existed between them until now, widening into a chasm that they will never be able to bridge. It is only now that each will realize they have no desire to do so anyway.