Original: Lennon 's inspiration for the song came when his son , Julian , showed him a nursery school drawing he called '' Lucy - in the sky with diamonds '' , depicting his classmate , Lucy O'Donnell .
Simplified: According to the Beatles , Lennon 's son , Julian Lennon showed his father a nursery school drawing he called Lucy - in the sky with diamonds , depicting his classmate , Lucy O'Donnell .