If you play around with an ultrasound field for long enough, you can set up standing waves that allow you to levitate water drops in mid-air -- but that's only where the fun starts.
The drops, held in a node of low pressure and supported in mid-air by a high-pressure zone sat beneath them, can be excited by different frequencies of ultrasound. When the frequency matches a multiple of their resonant frequency, they show weird modes of oscillation, forming stars with three, four, five or however many points -- the number matching the multiple of frequency. But all that science aside, don't they look pretty? [New Scientist]
(Oh, and excuse the music. Please, please, please, excuse the music.)