The title of Part II is "The Sieve and the Sand", which is a reference to a childhood memory about how he once tried to fill a sieve with sand "...because some cruel cousin had said, 'fill this sieve and I'll give you a dime!'" (Bradbury 78). Of course, since this is practically impossible to do but Montag was too young to know this, he became tearful, hot, and tired.
In the next paragraph, Bradbury discusses how Montag recalls this memory as he reads passages from the Bible as he rides on the subway: "...if you read fast and read all, maybe some of the sand will stay in the sieve" (Bradbury 78); of course, the words "fell through" as did the sand in the sieve. At this point, Montag seems to be struggling with his ability to retain the information from books.