Final answer:
Singing a note that matches the glass's natural frequency causes it to vibrate and shatter due to resonance and energy.
Explanation:
The performer must be singing a note that corresponds to the natural frequency of the glass. As the sound wave is directed at the glass, the glass responds by resonating at the same frequency as the sound wave. With enough energy introduced into the system, the glass begins to vibrate and eventually shatters.
Learn more about resonance and natural frequency here:
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