Why does thunder boom?

You ask: Why thunder boom?

Flash come first, boom come after — here why. Lightning is big spark jumping through sky. Spark so hot — five times hotter than the face of the sun — it cook the air in a blink. Hot air jump apart faster than sound, shove all the air around it hard. That shove make a big wave roll through air. Wave hit Caveman ear — BOOM. Long spark reach far, so sound from close part come quick, sound from far part come late. That why boom roll and rumble, not one clean clap. Light run fast, sound run slow, so Caveman see flash then wait for the noise. Cave wisdom: thunder just sky-air slammed apart by lightning, then rushing back together.