A little while back, I did a post on the children's show World World, in which I explained - among other things - why I probably ought not to be allowed to watch children's programs. (If you're unfamiliar with Word World, take a moment and read that post - you'll need the background for what come next.)
I think Firstborn is the same way. His brain just... goes places.
A couple of weeks ago, he's riding in the car with his mother, and says - out of nowhere - "I think in Word World... if you took the letters for 'nothing' and put them together... they'd turn invisible."
Which, if you think about it, makes perfect sense - but I'm guessing there aren't many nigh-five-year-olds out there who are concerned with the existential ramifications of a world where everything that exists is composed of letters.