How is thunder and lightning made, and what causes it?

Storm clouds have lots of tiny particles in them, and these are called hydrometeors. These are tiny particles of ice crystals. In the same way as if you rub a balloon on your head and it can stick to the wall because of static electricity, when hydrometeors bounce together, they rub charge on and off each other. This makes charge get carried up and down within the cloud. This creates a difference in charge across the cloud, creating an electrical field. The result of this is that the Earth has a different electrical charge to the cloud. When the charge difference becomes big enough, eventually the insulation of the air breaks down and you get a lightning strike. A lightning bolt carries between a billion and 10 billion joules of energy, which is enough energy to make 100 000 pieces of toast! The lightning bolt is sufficient to heat the air around it to about 30 000 degrees centigrade - that's about six times the surface temperature of the sun. When you make something that hot, the gas molecules get so excited that you literally rip them to pieces. The electrons in the gas molecules jump to a very high energy level and then jump back again to a normal energy level. When they do that, they give out light. The light that they spray out is what you see as a flash of lightning. The sudden extreme heating of the air causes a compression wave, like clapping your hands together, around the lightning bolt, and that's what slowly propagates to you as a roll of thunder. As light travels much faster than sound waves, that's why you see the lightning first and then you hear the thunder.