The opening riff of the Beatles' 1964 single "I Feel Fine" is frequently cited as the first use of feedback on an album. "I defy anybody to find an earlier record," John Lennon challenged in a 1980 Playboy interview, "unless it is some old blues record from the '20s." As it turns out, Lennon's caveat was necessary; blues musicians such as Johnny "Guitar" Watson had been using intentional feedback for years, but "I Feel Fine" was the first time it appeared in a popular rock 'n' roll song. In a 1994 interview, Paul McCartney said the effect was discovered by accident; Lennon absentmindedly leaned his guitar against an amplifier and liked the sound so much he used it in the song. The riff paved the way for later artists such as Jimi Hendrix and the Who, who took that screeching and made it their own. The Beatles would later pave the way again, becoming one of the first popular acts to use backwards tape loops in their 1966 song, "Rain." Some of these backwards messages would be used by conspiracy theorists as "proof" that Paul McCartney was dead.