What causes thunder and lightning?

Thunderstorms make lightning when ice and water inside a cloud collide and separate electric charges. When the difference grows too strong, electricity jumps through the air as lightning. That flash heats nearby air very fast, and the sudden expansion makes the rumbling sound called thunder.

The flash reaches your eyes before the sound reaches your ears because light travels far faster than sound. Counting the gap can help estimate distance: about five seconds between flash and rumble means the storm is roughly one mile away, so it is time to be indoors.