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

Answer:-

A portion 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 basically changes the pitch of the tone. The tone you hear is formed when the crystal vibrates at frequencies in the audible range for humans. Even though it's difficult to see the glass actually flexes during the vibrations. If the glass deforms adequately it can break as crystal is brittle and not awfully elastic. It isn't likely that a flawless glass scould be made to break merely by rubbing its rim.

The parable Busters demonstrated the resonance of a crystal glass and shattered several by exciting the glasses with a human voice.