My Last Line

I know I shouldn't do this, but I often can't help myself. Lots of times when I start a book, I flip to the last page and I read just the last sentence. Only the last sentence. Then I play a game with myself to guess how it gets from the first sentence to that last, usually inexplicable, sentence.

So, in honor of my own awful habit, and in honor of the rewrite that I'm just (*still*) finishing, here is the most recent last sentence of my book:

In an instant, Murph's room was engulfed in laughter and the chattering of two hundred tiny hot air balloons, each lending its own unique voice to the ocean of joyous noise.

So can anybody guess my theme?