In part 1, section 16 of Fahrenheit 451, when Millie can't answer the question, what does she get up to do?

Montag is still thinking about all that has happened since
the beginning of the story.  Clarisse's question to him about whether or not he is
happy, her suggestion that he might not be in love, the death of the old woman who would
not give up her books are all events that are weighing heavily upon Montag.  He asks
Millie if she can remember when and how they met ten years earlier because he cannot. 
She doesn't remember either.  Montag realizes how shallow their existence is; how devoid
of real emotion or love.  After Montag asks Millie the question, she gets up, goes to
the bathroom and Montag can hear her pouring a glass of water and then swallowing. 
He tries to count the number of swallows and wonders how many of those pills she had
taken already that night.  He knows that if she died from an overdose, he would not cry
because he doesn't really have a connection to her.  He is very saddened by the
emptiness.  Millie's actions reveal that she, too, is not happy.  She takes the pills to
avoid reality. Reality is cold and sterile in their world and that hurts, so Millie
hides behind the chemical relief of the pills.  She is like most of the people in that
society - escaping a harsh, cold reality for a less painful chemical
addiction.