For adolescent boys, a changing voice that cracks in the middle of a sentence can be a great embarrassment. Though embarrassing, a cracking voice is a natural part of adolescent development.
As a boy goes through adolescence, his secondary sex characteristics develop. One of these characteristics is the rapid growth of the larynx and vocal cords. A boy's voice deepens as his larynx develops because the bigger the vocal cords, the deeper the voice.
In fact, vocal cords are similar to other musical instruments in this regard. The longer the harp string, for instance, the lower the note it plays. Similarly, if you are blowing into a bottle to create a certain pitch, the larger the bottle, the lower the pitch. When it comes to voices, the bigger the vocal cords, the lower they resonate, and the deeper the voice will be.
But why does a boy's changing voice break and crack? For the same reason growing adolescents are often gangly and awkward. Because the brain is becoming accustomed to working with bigger body parts. Even for an adult, a consistent and even voice depends on the brain's ability to constantly monitor the sounds that come from the voice.
The brain can do this quite easily under normal circumstances. But, when a boy's vocal cords enlarge, the brain must relearn how to monitor and control the voice. A cracking voice is proof that an adolescent boy's brain hasn't become completely proficient at coordinating its careful monitoring of the sounds coming from the vocal cords.