Come on now, we're going to build a mirror factory first and put out nothing but mirrors for the next year and take a long look in them.
Sieve and the Sand
Granger suggests they build a giant mirror factory and take a look at themselves. Clarisse is also compared to a mirror for helping Montag to see himself. Mirrors represent internal reflection- seeing ourselves for who we really are.
The sand is knowledge that eludes Montag. The sieve is his mind trying to make the knowledge permanent.