The sentence can be completed as follows:
An example of resonance is when a long, high pitch causes a crystal glass to begin to vibrate and then break.
In fact, the glass has its own frequency of vibration. When there is a sound, normally nothing happens; however, if the frequency of the sound is equal to that of the glass, the glass starts to vibrate and the amplitude of its vibrations increases more and more, until the glass breaks. This phenomenon is called resonance.
