Why does thunder make a big sound?
For grown-ups: the science
Thunder is the big rumble made when lightning heats air so fast that the air suddenly expands and pushes through the sky.
A lightning channel heats surrounding air to roughly 30,000 K in a fraction of a second. The air expands explosively, creating a shockwave that travels through the atmosphere and is heard as thunder when it reaches the ear.
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Transcript
A bright lightning flash zigzags across the dark sky. What makes the giant boom that follows?
Lightning heats the air around it incredibly hot in less than a blink. The hot air expands outward all at once.
That sudden expanding air makes a strong moving push called a shockwave. It races through the air in every direction.
When the shockwave reaches your ears, it makes your eardrums move. Your brain hears that big rumble as thunder.
So thunder is the big sound of air suddenly expanding after lightning heats it. The moving push travels all the way to your ears.