When Montag complained about being unable to remember Mildred, what explanation did Granger give?
Toward the end of the story, on page 158-159 in my edition of "Fahrenheit 451." When Montag says he can't remember Millie, Granger tells him a story about the loss of his grandfather and how to remember someone when they die, they must leave something. He asks Montag "whet did the others give to each other?" Montag replies "Nothingness."
"Everyone must leave something behind when he dies, my grandfather said. It doesn't matter what you leave, but you must leave something." This is paraphrased, but that is what Granger told Montag.