A wine glass is placed in a (more or less) soundproof container. A speaker generates intense sound at a tunable frequency. When the resonant frequency is hit, the wineglass shatters. The whole thing is viewed with a strobe. As resonance is approached, large amplitude vibrational motion of the glass is clearly visible. (For a larger class, it can be viewed on the overhead television).
This is a slightly fussy demonstration, and the conversion between the cycles-per-second setting of the audio generator and the cycles-per-minute setting of the strobe can be confusing. It's best to have it completely set up in advance and then not touch anything until you're ready to go.