What causes lightning during a thunderstorm?

Lightning is caused by the buildup of electrical charges within a thunderstorm. As warm air rises and cools, it creates updrafts and downdrafts within the storm cloud. This causes collisions between ice particles, creating a charge separation: positive charges accumulate at the top of the cloud, and negative charges build up at the bottom. When the difference in charge becomes large enough, a discharge of electricity occurs, which we see as lightning. The rapid expansion of heated air around the lightning causes the thunder that follows.