called the Mellotron.
When I first heard "Strawberry Fields Forever," I hadn't paid much attention to an otherwise obvious drastic change in the music. The chorus begins normally, as expected, leading into the First Verse. By the time the Chorus returns, however, the pitches in John Lennon's voice becomes unusually lower, and the music speeds up.
The Mellotron was an analog organ organ that could loop various sounds via magnetic tape with the push of a key. It is used in many parts of this song, particularly the "flute-like" introduction. There was one problem with the Mellotron, though. Because it used magnetic tape, the length of the loop largely depended on the amount of tape available. Translation: You needed a LOT of it for long loops.
Video demonstrations of the Mellotron are readily available on YouTube for anyone curious to see how this machine. Also interesting is the Beatles "song," Revolution 9 from The White Album. It is really a combination of random sounds looped together. This surprised me because this doesn't sound like any of their previous music: not even the Sgt. Pepper album.