John Cage was not the first composer to alter a piano’s sound by placing objects between or on its strings. Maurice Ravel, for example, called for a piano with paper woven through its strings for his L’enfant et les sortileges (1920-25) in order to simulate the sound of a Luthéal. Cage was also influenced by Henry Cowell’s concept of the “string piano” in which a whole range of sounds could be coaxed from the instrument by manipulating the strings directly, rather than through the keyboard. Cowell’s The Banshee (1925), for example, called for the performer to attack the strings with finger tips, fingernails, and the palm of the hand to achieve different effects. Cage was able to combine these two ideas, using a variety of materials (bolts, screws, weather stripping, rubber) affixed to the strings to create an instrument that could be played from the standard keyboard but which sounded little like a piano.
In fact, Cage’s prepared pianos often sound more like percussion ensembles than pianos, and this characteristic is not coincidental. In 1938, Cage was employed as a composer and accompanist for dance classes at the Cornish School in Seattle and was asked to write music for a dance by Syvilla Ford. At the time, Cage had been writing mainly for percussion ensemble, and that was the medium he intended to use for the dance accompaniment. Unfortunately, the Cornish School’s stage was too small to fit a full percussion ensemble—he would have to write for piano. Using materials he had around his home (starting with a pie plate) Cage experimented until he had converted his piano into a virtual percussion ensemble under the control of a single player. The piece that emerged was Bacchanale (1938), Cage’s first work for prepared piano.
In 1947, avant-garde German director Hans Richter released an experimental film called Dreams that Money Can Buy, a loosely-narrated pastiche of episodes "based on drawings, objects, and suggestions by" celebrated Dadaist artists. Cage composed music for prepared piano for the Marcel Duchamp segment, "Discs."
The movie in full, with segments by Duchamp, Man Ray, Max Ernst, Alexander Calder, and Fernand Léger, can be found at The Internet Archive.