Relate Montag's childhood experience with the sieve and the sand to his difficulty reading and understanding his books.
The episode of the sieve and the sand for Montag in Fahrenheit 451 is comparable to his inability to understand what he reads.
Just as he couldn't fill the sieve no matter how fast he filled it with sand, Montag, no matter how much or how fast he reads, can't comprehend it, at least not at the time the narrator tells about the incident.
But it is also indicative of what Montag must try to do--read as fast and furiously as he can so that he can experience the reading before it is taken away. If he reads fast enough, maybe he'll fill the sieve, figuratively.
Montag is already trying to memorize what he reads while on his way to Faber, during the scene in which the memory of the sieve and the sand is revealed, but he keeps getting interrupted by a commercial.