Read the passage and answer. Lightning has been a mystery since early times. People of ancient civilizations believed angry gods threw lightning bolts from the sky. Nobody understood that lightning resulted from electricity until Ben Franklin flew a kite with a key dangling from the string, and it was struck by lightning. In current times, it is known that lightning has a very scientific cause. Generally, within a storm cloud, friction from water and ice-laden clouds creates a negative charge at the bottom of the cloud. When that charge grows too great for the air to hold it back, it is united with a positive charge from the Earth, creating a channel of electricity that flows between the two points. The charge remains invisible as it moves towards the ground until it meets the charge rising from the ground. Once they meet, a fifty thousand degree current superheats the air around the channel, resulting in an explosion of sound known as thunder. In fact, very recently it has been discovered that occasionally the positive charges appear at the bottom of the cloud, which are then met by negative charges from earth. The author indicates that thunder is created when —.