While riding on the subway to get to Faber's house, Montag decides that he needs to inundate himself with reading material; he will have to give the book back to Beatty soon, and he has no place to go where he can calmly absorb the material in it, and remember it clearly. Instead, like the memory of a sieve being filled with sand from when he was a child, he can only hope that by cramming, a few bits and pieces of the book will stick with him.
He is interrupted by a loud commercial for "Denham's Dental Dentifrice", which he attempts to ignore but cannot, because it is so intrusive. The commercial seems to have a life of its own, from the way Montag reacts to it, as if it is consciously trying to distract him as much as possible. Eventually Montag appears to lose his mind a little bit, yelling "shut up!", which shocks and unnerves the other passengers; they think he's crazy.
The spooky thing about the passengers was that they were going right along with the commercial, tapping their feet and mouthing the words as though it were perfectly normal to be so thoroughly engrossed and commanded by a commercial.