In Fahrenheit 451, what happens with the Mechanical Hound after Beatty speaks with Montag during Montag's illness?
Although Beatty suspects that Montag has stolen more than one book, he is not certain; when Montag is ill and Beatty visits, Beatty sees that the curiosity of books has taken hold. This is dangerous; if people are allowed to read, they will develop unusual opinions and positions, and cause strife in the controlled society. Beatty explains to Montag that it is unofficial policy to allow firemen twenty-four hours with one book, to clear their heads, but since he is suspicious, he send the Mechanical Hound to Montag's house to help put fear into him. Later, he explains:
"Well," said Beatty, "now you did it. Old Montag wanted to fly near the sun and now that he's burnt his damn wings, he wonders why. Didn't I hint enough when I sent the Hound around your place?"
(Bradbury, Fahrenheit 451, Google Books)
His implication is that even if Montag had stolen many books, he only had to turn himself in and his curiosity would be excused. While Montag and Mildred are reading, they hear scratching outside, and Montag is concerned that the Hound is there, but he believes himself to be safe as long as he turns one book in. Essentially, Beatty tried to use the Hound as a last-minute deterrent against keeping the books, knowing both that Montag is already scared of the Hound, and that if there are books in Montag's house, someone will turn him in.