In an interview with BBC's Zane Rowe, Sir Paul revealed that the band added a sound to "A Day In The Life" which only dogs would be able to hear: a dog whistle, in fact. It's a 15-kilohertz tone which sits above the range of human hearing but which is perfectly audible for our canine friends.

McCartney made the revelation when discussing the merits of vinyl. "I think vinyl is the best," he said. "It just sounds good. I asked my engineers why it sounds good and they explained that there are frequencies above and below that you can't hear. We got into a rap with George Martin a long time ago. We'd talk for hours about these frequencies below the sub that you couldn't really hear and the high frequencies that only dogs could hear. We put a sound on Sgt. Pepper that only dogs could hear. If you ever play Sgt. Pepper watch your dog."