what is the meaning of the tiltle of part two of this novel ?

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 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. This frustrated Guy, causing him to cry. The other incident involves Montag's attempts to memorize. He is trying to commit to memory the Book of Ecclesiastes from the Bible, but the jingle from the advertisement for Denham's Dentrifice that keeps playing while Montag is on the train is interfering and the words to Ecclesiastes fall through his memory just like the sand went through the sieve all those years ago. Just as he was as a child, Guy Montag is frustrated at his inability to hang onto the words he's trying to memorize. Guy's modern world counts on this inability to concentrate. This world he lives in without books has encouraged people to live for the immediate moment; it's a world of sound bites and expediency. By filling every place with mindless sound such as the advertisement jingle, people can't concentrate and do any serious thinking. If people can't think, they are much more easily controlled. This is just where society, and the government in the book, want people. By banning books, people's minds have been turned into sieves unable to hold thought.

Also, adding onto what Aslan has said.........the government is hypocritical in the sense that they allow themselves to read a bit in order to obtain knowledge, therefore, giving them power and authority over society. However, despite allowing themselves to obtain that knowledge from books, the government restricts the rest of society from obtaining knowledge because they want control over society. So the sieve and the sand can also represents the individual's mind in the sense that they have no ability to think for one's self, because the government thinks for them. Therefore, they have no purpose in life if there is no self growth, just like a sieve doesn't have a useful purpose if sand is sieved through it.