What does the sieve represent? What is the sand a symbol of? Describe how this memory relates to Montag’s current situation

As Montag rides the subway to Faber’s home, he has a memory: Once as a child he had sat upon a yellow dune by the sea in the middle of the blue and hot summer day, trying to fill a sieve with sand, because some cruel cousin had said, “Fill this sieve and you’ll get a dime!” And the faster he poured, the faster it sifted through with a hot whispering. His hands were tired, the sand was boiling, the sieve was empty. Seated there in the midst of July, without a sound, he felt the tears move down his cheeks.