I have to blow on it, it might be old.

My son is a very astute kindergartener,
but sometimes he baffles me.
He goes with his class every Friday to the Library at school and they get to bring a book home.
We have been reading a lovely book he picked out called "The Bunny Book" by Richard Scarry.  I vaguely remember the picture on the cover of the book from when I was young.
Tonight he read me "The Bunny Book" again and afterwards he closed the book and blew on it.
(I had seen him do this the night before, also)
I said, "we probably shouldn't blow on that book since it is borrowed and we really don't want to spread germs"
His reply was, "but I HAVE to blow on it, it might be old"
Finding no possible connection to having to blow on old things, aside from blowing into Nintendo cartridges, I inquired, "I don't understand, sweetie, why would you need to blow on it because it's old?"
"Because that's what they do in the movies", he replied.
"OH!" I exclaimed, "I see", trying not to giggle, I explained how the old books in the movies have been sitting a long time and are dusty and THAT'S why they blow on them!