The following text is for questions 16 to 20.
 

    The conditions needed to produce lightning have been known for some time. However, exactly how lightning forms has never been verified, so there is room for debate. Leading theories focus around separation of electric charge and generation of an electric field within a thunderstorm.

    A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm. As the differences in charges continue to increase, positively charged particles ascend to taller objects such as trees, houses, and telephone poles. A channel of negative charge, called a "stepped ladder," will descend from the bottom of the storm toward the ground. It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, positive charges form on the ground and in objects on the ground. This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer." When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning stroke, if enough charge is left over, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.

(Source: http://www.srh.noaa.gov/jetstream/lightning/lightning.html)

What makes lightning flicker?