How does a thundercloud make lightning?

Lightning is a giant electric spark between two regions with opposite charges: inside a cloud, between clouds, or between a cloud and the ground. The charges build up inside a storm cloud through countless tiny collisions. In the middle of the cloud, rising air carries small ice crystals upward while heavier hail hangs or falls. When they bump into each other, the light ice crystals become positively charged and ride up to the top of the cloud, and the heavy hail becomes negatively charged and stays lower down.

At first, air acts as an insulator between the charges. But when the difference grows too great, the air's insulation breaks down and electricity rushes through it in a flash. The channel heats the air to about 30,000 °C, and the air expands so violently that it makes a sound wave: thunder. Because sound travels about a mile every five seconds, you can count the seconds between flash and rumble, divide by five, and get the distance in miles.

People did not always know lightning was electricity. In 1752 Benjamin Franklin proposed flying a kite into a storm to find out, and experiments like his showed that lightning and electricity are the same thing. It was dangerous work: in 1753, Georg Wilhelm Richmann was killed in Saint Petersburg while trying to repeat the lightning experiments.