Boulez then tried out new ideas. He wrote works which would sound different at each performance because the players could improvise some ideas. "Pli selon pli" ("Fold upon fold") was a work for orchestra and soprano. It was based on a poem by Stéphane Mallarmé about a swan that could not fly away because it was stuck on the frozen lake. His piece "Eclat" (1965) also uses improvisation. The conductor is free to decide on how certain bits could be played and makes signs to the players to improvise in certain ways that the composers has written down. This kind of composition is sometimes called “Chance music” because it is different each time it is played. The American composer John Cage also experimented with Chance music.