A lightning bolt is created from the ground up and also down from the sky. Negative charges from a thunderstorm slowly lower to the ground, while positive charges rise from the ground or an object toward the sky. When these two streamers, or ion groups, meet, visible lightning is created.
The rising and lowering streamers are invisible to the human eye, and only become visible when they meet, which causes a lightning bolt from the grounded object to shoot toward the sky. This occurs in a split second, making it nearly impossible to determine from what direction the bolt came. The upward streamer normally forms from the tallest nearby object, but sometimes a smaller object becomes the source of the streamer.