I was sitting in my car looking at a stop sign the other day, and all of the sudden it started shaking uncontrollably. Immediately I knew it must be a child shaking it, as there was simply no other possibility. No other person would just stand there and shake a street sign for no purpose beyond its' own sake. There is no benefit, no reward, no ultimate reasoning beyond 'I want to shake this street sign". As I looked down, I was right, and the kid was grinning like crazy.
We lose the ability to think like that as adults. It's not even that we have the thought and then consciously decide against it; we're incapable of even conceiving it. To just 'play' in life. And I think it's the most important aspect of our humanity. It is the very genesis of all our innovative and creative ideas, yet we treat it as something to be expelled in the process of "growing up".