What is Lightning?
When the sky is dark and angry and it is raining or about to rain, you sometimes suddenly see a zig-zag flash of lightning in the sky and then a tremendous ripping sound and a crash of thunder!
The lightning we see in the sky is caused by electricity. Each tiny drop of water in the rain-cloud has a little charge of electricity. There are billions of water drops in every rain-cloud, so the entire cloud has a powerful charge of electricity. When two clouds with electric charges come near each other, electricity rushes between them. The flash we see is really a huge electric spark.
The flash of lightning heats up the air around it. The heated air goes rushing out in all directions. It hits the cold air, making it shake. This is what causes the crash of thunder.