Part two of Farenheit 451, entitled "The Sieve and the Sand" creates a metaphor between the childhood incident of Montag trying to fill a sieve with sand quickly enough to win a dime and ultimately crying in frustration to the point where he is now in the novel as he attempts to skim through the Bible on the subway, hoping desperately that some of the passages will stick in his memory.
Like the title of Part One, "The Hearth and the Salamander," Bradbury chooses a title for part two of the novel that also connects to the elements with "The Sieve and the Sand," creating an image of earth, but one that is transitory--fleeting like Montag's sand slipping through his sieve; Bradbury's powerful use of imagery reinforces his theme of change and the idea of the 'temporary'--nothing can last forever.
Here is an overview of the novel:
The meaning of the title of Part Two is that the "Sieve and the Sand" refers to two incidents: one from Montag's childhood and one from the present. In the incident from his childhood, a cruel cousin challenged Guy Montag to fill a sieve with sand in exchange for a dime. Of course, the more sand that the child Montag put into the sieve, the more sand fell through the holes in the sieve. His hands were tired, the sand was boiling, and the sieve was empty. This frustrated Guy, causing him to cry. The other incident involves Montag's attempts to memorize. He was trying to memorize the Bible line by line, but the noise from the advertisement for Denham's Dentifrice that keeps playing while Montag is on the train is interfering and the words to the Bible fall through his memory just like the sand went through the sieve all those years ago.