In Ray Bradbury's novel, Fahrenheit 451, as Montag is floating down the river, having evaded the helicopters and The Hound, he starts thinking about the sun. In fact, since he met Clarisse, he has been thinking about many things, more than ever before. As he glides along in the water, he imagines in his mind's eye, how the sun burns: it burns Time, he concludes. He imagines the sun burning every clock. After a time, Montag, once a fireman, comes to a realization as to why he cannot burn anything ever again.
The sun burnt every day. It burnt Time. The world rushed in a circle and turned on its axis and time was busy burning the years and the people anyway, without any help from him. So if he burnt things with the firemen and the sun burnt Time, that meant that everything burnt.
He realizes the burning has to stop: either he has to stop or the sun. Since the sun will not stop, Montag and other firemen will have to do so, so the world can begin again, fresh.
Why does Montag start to think about things that burn? For the first time in many years, Clarisse looked at him and saw him; she heard him. Instead of feeling one-dimensional, he started again to feel alive. Reading allows this to happen to him as well, as he uses his mind to think and try to make sense of what he has read. He is forced to find Faber to help him change. As Montag becomes a reasoning, thinking human being, he cannot help but contemplate things that he has not had to think about before. He also has to learn who he is, but must do so by thinking about who he has been.
As he floats along in the water, perhaps the symbolic burning within him for change brings to mind the concept of burning, and he begins to look at it from a new vantage point. And he realizes that in order to survive this new life ahead of him, he must change first. Though the sun will not change, men can, and he believes he must do what is in his power to effect change where he is capable of doing so, hoping that others will follow.