Why does a glass filled with water shatter when you run your finger along the rim of the glass?

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September 04, 2007 3:59AM

A piece of crystal stemware has a natural or resonant frequency

that can be excited by rubbing a moistened finger along its rim.

Filling the glass with water merely changes the pitch of the tone.

The tone you hear is created when the crystal vibrates at

frequencies in the audible range for humans. Although it's

difficult to see, the glass actually flexes during the vibrations.

If the glass deforms sufficiently, it can break, since crystal is

brittle and not very elastic. It is not likely that a flawless

glass could be made to break merely by rubbing its rim. The

MythBusters demonstrated the resonance of a crystal glass and

shattered several by exciting the glasses with a human voice. One

professional singer broke a glass without the use of amplification.

See the video.

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