the realization that each random passerby is living a life as vivid and complex as your own -- populated with their own ambitions, friends, routines, worries, and inherited craziness -- an epic story that continues invisibly around you like an anthill sprawling deep underground, with elaborate passageways to thousands of other lives that you'll never know existed, in which you may appear only once, as an 'extra' sipping coffee in the background, as a blur of traffic on the highway, as a lighted window at dusk.
something as innocent as the flutter of a butterfly wing could result in a typhoon on the other side of the world.