In his song "I'm a Child" Neil Young asks this question and it has been puzzling fans for decades now. Lots of people think that the answer to that pseudo-riddle is 'dark black' , but everybody who has cleaned out a fireplace or a BBQ knows that the answer is ash colour or even white. :-)
Of course, the answer doesn't matter, the question is: why does Neil ask this question in a song about childhood? I always thought that it was an example of one of those difficult questions children can ask leaving the parent baffled, but maybe you have a different explanation.
I am a child, I'll last a while.
You can't conceive
of the pleasure in my smile.