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Acousticians from the Polytechnic Faculty of Mons measured the precise resonant frequency of crystal glasses. They were reproduced by a frequency generator and played through a loudspeaker in the presence of each of the glasses. At this frequency, the sound energy is trapped in the glasses, which then start to vibrate. The vibration finally causes them to break. This phenomenon, invisible to the naked eye, was filmed by a camera that recorded 5,000 images per second. The film was then screened at a speed of 25 images per second in order to make the distortion of the glass and its explosion visible.