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