The smaller denser falling drops have negative charges. When the buildup of electrical energy between the large higher positive droplets and smaller negative charged areas in the cloud reaches a peak, we have inner-cloud lightning. Continual up and down drafts creates constant mixing of the charges resulting in repeated lightning and thunder. Lightning that strikes the earth consists of one or more electrical discharges called strokes. The bright light that we see in a flash of lightning is called a return stroke.
Air heated by return strokes expands quickly producing wave of pressure called thunder.
Prehistoric people thought that thunder was the sound of the gods roaring in anger. Today we know that thunder is caused by the violent expansion of air that has been heated by lightning. Air is heated instantly when an electric charge of lighting passes through it. The heat causes the molecules of air to expand, or fly out, in all directions. As the molecules seek more room, they collide violently with layers of cool air, and set up a great air wave that has the sound of thunder.