Watch the moment Paul McCartney composed "Get Back"

In one of many magical moments in Peter Jackson's documentary The Beatles: Get Back, Paul McCartney writes "Get Back."

"We were sitting in the studio and we made it up out of thin air… we started to write words there and then… when we finished it, we recorded it at Apple Studiosand made it into a song to roller-coast by," McCartney said in The Beatles Anthology.