I find it weird that we don’t know what causes lightning.
There are two hypotheses noted by Wikipedia:
Cloud particle collision hypothesis
According to this cloud particle charging hypothesis, charges are separated when ice crystals rebound off graupel. Charge separation appears to require strong updrafts which carry water droplets upward,supercooling them to between -10 and -40 °C (14 and -40 °F). These water droplets collide with ice crystals to form a soft ice-water mixture called graupel. Collisions between ice crystals and graupel pellets usually results in positive charge being transferred to the ice crystals, and negative charge to the graupel.[14]
Updrafts drive the less heavy ice crystals upwards, causing the cloud top to accumulate increasing positive charge.Gravity causes the heavier negatively charged graupel to fall toward the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate a lightning discharge, which occurs when the distribution of positive and negative charges forms a sufficiently strong electric field.[14]
Polarization mechanism hypothesis
The mechanism by which charge separation happens is still the subject of research. Another hypothesis is the polarization mechanism, which has two components:[34]
- Falling droplets of ice and rain become electrically polarized as they fall through the earth’s magnetic field;
- Colliding/rebounding cloud particles become oppositely charged.
There are several hypotheses for the origin of charge separation.[35][36][37]
Here’s a cool video of lightning captured at 7,207 images per second. Light travels at something like 300,000,000 meters per second so it doesn’t break down the ‘big flash’, but it shows a lot.
Here’s another quote:
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.
Not Unreasonable 
Our eyes only see the return path. Would be a great view from space (minus the clouds).