Final answer:
The song 'Lucy in the Sky with Diamonds' by John Lennon opens with a piano, not a synthesizer, setting the psychedelic tone for the track.
Step-by-step explanation:
The instrument that starts John Lennon's 'Lucy in the Sky with Diamonds' is not a synthesizer, but rather a piano. The introduction of the song features a distinctive piano motif that sets the tone for the psychedelic atmosphere of the track. It's neither the guitar string, the air inside the guitar, the air surrounding the guitar, nor the ground beneath the musician that is featured in the beginning; it is the piano that leads into the song.