All objects have a resonant frequency, the natural frequency (or frequencies) that makes them vibrate. That's why a really, really good opera singer could theoretically shatter a wineglass with a particularly high note. They'd just have to hit the resonant frequency.

Even googly eyes have a resonant frequency. One intrepid experimenter found it, making a collection of googly eyes go even more googly-eyed than normal, vibrating back and forth in a sort of plastic rapture. 

Resonant Frequency of Googly Eyes. from Rob Cruickshank on Vimeo.

That frequency happens to be 433 Hz. Useful to know in case you ever want to make your craft project go cross-eyed. 

[h/t: io9]