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How do you prevent my voice from breaking while i present a speech?
Unfortunately there is nothing much you can do to prevent your voice from cracking. It is a natural part of going through puberty, and will be over in a few months, but in the meantime you just have to suffer through it.
your spine has controls the pressure between your bones so it doesnt't smash up together and also your muscles help by filling in the space between the bones.
speak clearly,stand straight don't slouch, speak loudly, have eye contact, don't face downward...no need to look at your shoes, and that's bout it. all the rest is up to the quality of the speech