A prepared piano is a marvelous instrument. It's hardly a piano anymore, really. You take a normal piano and alter its sound by placing objects (preparations) between or on the strings or on the hammers or dampers. The wonderful thing about this is that all it takes are some screws, washers, pie pans, or any other object you have lying around that you can stuff into your piano, to transform a mild mannered instrument into a super piano (or "supiano", if you will).
Having coined the term "prepared piano" himself, John Cage first prepared a piano when he was commissioned to write music for "Bacchanale", a dance by Syvilla Fort in 1938. He had been writing exclusively for a percussion ensemble, and then someone was so kind as to casually mention that the hall where Fort’s dance was to be staged had no room for a percussion group. In fact, the hall was exceptionally small and the only instrument available was a single grand piano (minor detail, right?). After thinking about it, Cage said that he realized it was possible “to place in the hands of a single pianist the equivalent of an entire percussion orchestra ... With just one musician, you can really do an unlimited number of things on the inside of the piano if you have at your disposal an exploded keyboard.” Exploded keyboard... yeah, no problem, right? So Cage prepared this single grand piano and even quipped that by preparing it, he left it in better condition than he found it. Not sure what the owner of said piano thought...
In Cage's use, the preparations are typically nuts, bolts, and pieces of rubber to be lodged between or entwined around the strings. Some preparations make duller sounds, while others create sonorous bell-like tones and the individual parts of a preparation such as a nut loosely screwed onto a bolt will vibrate themselves, adding their own unique sound. Often, the pianist would be instructed to pluck and scrape the strings of the piano directly, a technique that Cage himself said was inspired by Henry Cowell's experiments with the so-called string piano (and I thought all pianos had strings... silly me). And in the end, it really does sound like an entire percussion orchestra.
The first time I heard a prepared piano, I would never have guessed that, well, for one that I was hearing a piano, and for another, that a single musician was producing all of the sounds I was hearing. In addition, watching a pianist play a prepared piano is just so much more interesting. One can't expect any of the sounds emanating from the instrument -and I wonder if the pianist even knows what it will sound like- and the way the musician plays, banging on the keys with fingers and fist, reaching inside to pluck the strings, well, it's all very energetic and exciting.