Tom Petty has opened up about his classic track "Free Fallin'" and says it was actually written as a joke for ELO frontman Jeff Lynn.

Petty says that tune was initially an afterthought until Lynn made one crucial suggestion.

"I was playing the keyboard and I just happened to hit on that main riff, the intro of the song, and I think Jeff said something like, 'That's a really good riff but there's one chord too many,' so I think I cut it back a chord and then, really just to amuse Jeff, honestly, I just sang that first verse," said Petty.

"Then he starts laughing."

"Honestly, I thought I was just amusing Jeff but then I got to the chorus of the song and he leaned over to me and said the word, 'freefalling.'

And I went to sing that and he said, 'No, take your voice up and see how that feels,'" said Petty.

"So I took my voice up an octave or two, but I couldn't get the whole word in.

So I sang 'freeee,' then 'free falling.'

And we both knew at that moment that I'd hit on something pretty good.

It was that fast."