This demonstration requires a fair amount of equipment. A stereo amplifier, whose output goes to a horn driver near the wine glass to be broken, can be switched between a frequency generator, easily tuned through a broad spectrum, and a frequency synthesizer which can generate a?very accurate frequency. The response of the wine glass to the sound is monitored with a microphone connected to an oscilloscope. The first four steps can be prepared before the lecture, but most instructors like to run through them with the class so the students can see the entire operation.
We have the Tacoma Narrows Bridge collapse on videodisk. There is also a good online version which you can link to. The link is clickable Tacoma Narrows Bridge Story http://youtube.com/watch?v=ASd0t3n8Bnc [1]
The fundamental resonance of a wineglass is the bell mode which can be seen in the strobed video above. Two wavelengths of the resonant frequency fit around the rim, travelling in opposite directions, constructively and destructively interfering. There are four nodal points along the rim of the glass which don't move. In between the nodal points the glass walls are moving back and forth. Sometimes when the glass shatters it leaves evidence of the nodal structure as seen in the gallery below. The last two are rare breaks.
Links:
[1] http://youtube.com/watch?v=ASd0t3n8Bnc