How does lightning form?

How does lightning form? Evidently we’re still trying to figure it out! It all starts in the clouds where both ice crystals and hail stones form: 

Scientists believe that as these hail stones fall back through the rising ice crystals, millions of tiny collisions occur. These collisions build up an electric charge which is stored in the cloud like a battery. ”A cloud is very much like a battery, but a battery with a much higher voltage than your typical flashlight battery… not 1.5 volts but 100 million volts.”

But what scientists don’t know is exactly how this electric charge generates lightning. “What remains a major meteorological mystery is how it is that ice particle collisions result in the generation of lightning. We’re very much in the middle ages on that problem.”

From the Discovery Channel’s “Raging Planet” series.