The Child is the Father of the Man

Since this is a family blog, I decided to link to this video:

As the commentator on the host site said, if this doesn't put a smile on your face, nothing will.
The video reminded me of the quote from Wordsworth (the title of this entry, and probably my
favourite Wordsworthian line).
It promts the following question from me: why does the baby laugh? What triggers it? etc.