What causes lightning?

The answer

Lightning forms when ice particles collide inside cumulonimbus clouds, transferring electrical charge. Smaller ice crystals carry positive charge upward while larger hailstones carry negative charge downward, creating a massive voltage difference - sometimes hundreds of millions of volts. When the electric field becomes strong enough to overcome the insulating properties of air, a conductive channel forms and electricity discharges as lightning, heating the air to around 30,000 Kelvin and causing the explosive expansion we hear as thunder.