Why Can a Prepared Piano Sound Like a Percussion Ensemble?

A prepared piano is not a different kind of instrument. It is an ordinary grand piano with screws, bolts, rubber or other materials placed between selected strings according to the score. John Cage developed the method around 1940 for a practical reason: the dance venue had no room for the percussion instruments he wanted, but it did have a piano.

The objects are not dropped in at random. Their material, size and point of contact alter how a string vibrates. One note may become short and dry, like a woodblock; another may acquire a metallic rattle. Some pitches remain recognisable, while others become complex bands of noise. Adjacent keys therefore cease to share one uniform piano colour and begin to resemble a small percussion ensemble. Preparation instructions must be followed precisely and without damaging the strings or soundboard.

The point is not simply to make the piano sound strange. It changes the performer's listening task. On an ordinary piano, attention readily goes to melody, harmony and touch. On a prepared piano, the material, attack and decay of each sound become part of the structure. The keys still control the sounds, but they no longer guarantee the familiar piano tone. The instrument becomes not only a keyboard of fixed pitches, but a set of vibrating materials that can be reorganised.

https://www.smithsonianmag.com/arts-culture/how-composer-john-cage-transformed-piano-with-household-objects-180973135/
https://www.musicaviva.com.au/news-stories/breaking-good/
https://www.secondinversion.org/2016/05/11/inside-john-cages-prepared-piano-qa-with-jesse-myers/