How do lightning strikes get started?

Inside thunderclouds, rising and falling air currents move ice particles and water droplets around, separating positive and negative charges. This creates a huge electrical potential difference. A stepped leader of charge moves downward from the cloud, and when it connects with an upward streamer from the ground, a bright return stroke flashes along the channel. That return stroke is what we see as lightning. The rapid heating of air around the bolt creates thunder, which is why lightning and thunder usually occur together.