In Part 2, Bradbury uses the metaphor of the sand and the sieve to explain Guy's futile-seeming hope that he can read and retain the entirety of the Bible:

Once as a child he had sat upon a yellow dune by the sea in the middle of the blue and hot summer day, trying to fill a sieve with sand, because some cruel cousin had said, “Fill this sieve and you’ll get a dime!” And the faster he poured, the faster it sifted through with a hot whispering. His hands were tired, the sand was boiling, the sieve was empty. Seated there in the midst of July, without a sound, he felt the tears move down his cheeks. Now as the vacuum-underground rushed him through the dead cellars of town, jolting him, he remembered the terrible logic of that sieve, and he looked down and saw that he was carrying the Bible open. There were people in the suction train but he held the book in his hands and the silly thought came to him, if you read fast and read all, maybe some of the sand will stay in the sieve.

This metaphor is also a flashback, and it contains vivid imagery of the hot beach and young Guy's despair. Guy himself makes the flashback into a metaphor for his attempt to understand and appreciate literature when books are not meant to exist. He has very little time and safety, so much to read, and no guarantee it'll stick in his mind. Guy is of two minds in this moment. He believes reading the Bible will be futile, and yet he's tempted to try it anyway, to see if "some of the sand will stay in the sieve."