The iPhone is many things to many people: a communicator, a Web browser, an e-mail reader, and more. Now, thanks to one of the most popular iPhone apps you can buy (for a mere 99 cents), it's also a musical instrument, as demonstrated in a clever YouTube video that shows a consort of musicians performing Led Zeppelin's "Stairway to Heaven." As the camera zooms in on what you think are woodwind players, you see that the instruments they're fingering and blowing into are iPhones! The remarkable Ocarina app, from SonicMule (Smule, for short) transforms this most up-to-date of contraptions into an instrument that dates back 12,000 years by harnessing the iPhone's built-in features in a wonderfully creative way.

We've become accustomed to iPhone applications making clever use of the device's touch screen, microphone, speaker, motion-sensor, and location-tracking ability. But Ocarina takes advantage of these features in a unique way. Blowing into the mic produces pitches determined by the combination of four "holes" (displayed as blue circles on the screen) you press. And how hard you blow affects the loudness, as with a traditional wind instrument. That lets you infuse your music with a degree of expressiveness, as does tilting the phone, which changes the timbre. Finally, the four holes allow for 16 combinations, each representing different pitches—enough for any song.

Some people have trouble producing sounds at first, especially before they realize that the microphone is off center (a small arrow on the screen shows you where it is) and that they don't have to blow very close to it. Once you've figured that out, try the tutorial on the Web site if you want to do more than just tooting. The instructions explain a very simple notation system for the instrument, and there's an active community of forum members who post song scores. After not too much practice, I was able to perform a rendition of Bob Dylan's beloved "Blowin' in the Wind" that was at least recognizable (though the community-contributed score's rhythmic notation left something to be desired.) If the Web isn't handy, an info page in the app can get you started.

You can choose the starting pitch and you can also set the mode (the note arrangement in a scale) to any of the traditional ones. Choices include Ionian (the standard Western major scale), Aeolian (minor), Lydian, Dorian, and the little-used Mixolydian. There's even a Zelda mode, for fans of that fantasy game. You can arrange the holes in a trapezoidal configuration (which looks a bit more traditional than the default rectangle), change their color, and alter the hue of the rings that flow across the screen from the microphone when you blow.—Next: Listening to the World