The title of section two of Fahrenheit 451 provides its central metaphor that has contrasting significance to both Montag and Faber, whose relationship this section develops. "The Sieve and the Sand" refers directly to a memory that comes to Montag as he rides the subway to confront Faber, trying to concentrate on reading the Bible and remembering its lessons in the incessant noise of the ubiquitous advertisements.
Montag tries to resist the mind-numbing jingles and oppressive propaganda by taking recourse in the truth and wisdom of books thought extinct. But he finds that when he struggles to focus on the meaning of Jesus's "Sermon on the Mount," with its lesson to be faithful and at peace like the lilies of the field, the words get lost amid the cacophony on the subway. The futile effort makes him recall a childhood experience at the beach when an older cousin tormented Montag by promising him a dime if he could fill a sieve with sand. Up until this point, the conflicted Montag has been looking for new meaning and purpose in books, and his personal struggle to liberate himself with their knowledge mirrors the stakes and implications for society at large.
The metaphor of the "porous" sieve is extended to Faber, with his conviction that it is the "texture" and "quality" of books that gives them their forbidden power. To Faber, books are dangerous because they "show the pores in the face of life," the unadulterated truth, blemishes and all.