system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What is a voice crack?

I was wondering what a voice crack is and how it happens.
gpt
An uncontrolled switch of vocal registers, unless you're someone like SIA or Mariah Carey, which are known for using voice-cracks as a way to color their singing.

Too much force going up, you're going to crack because your "chest-voice" (the darker and deeper part of your voice) can't go that high without switching gears. Too breathy or light when going down from your head-voice (the brighter and higher part of your voice) and you're going to crack as well because it's quite difficult to have a breathy and bright lower register, specially without switching back to your lower-gear.

Yodelers, for example, have mastered this switch and use it stylistically for their singing. 

A vocal crack is not unhealthy either, which means that if you crack, it's simply your voice choosing the best mechanism to produce whatever sound it was that you tried to make. It's also quite common for teenage boys because our voices start to get deeper, (most of us that is) due to puberty, which means that whatever note you could hit with your chest-voice before puberty, you're probably not going to be able after puberty finishes unless you have a solid-grasp of how your voice works. The cracks go away after puberty (usually) because you' eventually and subconsciously adjust your entire speaking-range to your new post-puberty range.

Hopefully all this answers your question.