Have you heard a big boom in the sky? That sound is called thunder.
Thunder happens during some rainy storms. You might see a bright flash first.
That bright flash is lightning. It is a quick, bright spark in the sky.
Lightning makes the air get very hot. The hot air grows fast, like a balloon.
Then the air cools and squeezes back. The air bumps and shakes.
Those bumps make a loud sound. That loud sound is thunder.
Sometimes thunder is a small rumble. Sometimes it is a big boom.
If you hear thunder, stay inside with a grown-up. Then you can listen, safe and calm.