In "Fahrenheit 451", Montag's reaction to the commercial on the subway is a turning point in his life. How does he react and why?

As Montag attempts to read and remember the Book of Ecclesiastes while riding on the train to see Faber, his newfound teacher. He cannot, however, manage it because the train's sound system plays an advertisement for Denham's Dentifrice over and over: "Denham's does it" with a bouncy jingle that interferes with his ability to think and remember. Montag begins to realize that wherever her he goes in his society, the system is there to limit and shape what he thinks by feeding him sights and sounds. He becomes more determined that ever to save books. After he gets to Faber's, Montag suggests sabotaging firemen by planting books in their houses. He and Faber also decide to make copies of the Bible which Montag had stolen from the fire the previous night. Finally, Montag will return to the fire station as a spy with Faber monitoring and analyzing the situation through the use of a two-way listening device placed in both their ears.