What's the riddle in "The Riddle"?

I listen to the radio a fair amount and the song "The Riddle" [lyrics] [block that page's pop-ups with Google Toolbar] by Five For Fighting has come on frequently of late. Now, I don't often think about songs I hear on the radio, but last time I heard this one I started thinking about the lyrics.
"The Riddle" as a whole seems to be (and is, according to FFF's myspace page) a message of love from a father to his son, painting a picture of close personal moments with their children that I think many fathers aspire to...

Yet he also touches unflinchingly on the personal. The CD's debut single "The Riddle" is a song he wrote for his children, while the companion video features his beloved blue Mustang (a car passed down to John from his father and the inspiration for the song "65 Mustang). Says John of the single, "A lot of my songs touch on mortality, but at its heart it's a love song from a father to his son." [link]

This aspect of the song nearly brings tears to my eyes, because it reflects a part of the father I hope someday to be. While this is a nice explanation of the meaning as a whole, I still can't help but get put off by the shallowness of some answers to the question "What's the riddle?"
So I ask: Is there a riddle other than pondering "the reason for the world" contained in the song? And if so, is there an answer deeper than "You and I"? And if not, what did some of those other 100 drafts look like?