system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does a sonic boom occur when you break the sound barrier?
gpt
A sound is a wave traveling through the air. If you’re traveling through the air, the sound races out a head of you, traveling at the speed of sound. 

If *you’re* traveling at the speed of sound, it can’t get ahead of you because you’re keeping up with it. But you’re also generating more sound. So all of that sound is just piling up on top of itself until you get a nice big boom as you’re passing through that wall of built up sound.