Goodnight what?

Ever since Gabriel was about nine months old I've been reading Goodnight Moon to him at bedtime.  It used to make an appearance about every 3 or 4 nights.  Now we try to read it at least twice a month. 

Well, once he started understanding words (somewhere around his first birthday), he started pointing to things in the book as I read along.  When we got towards the end, to the two pages that say, "Goodnight stars.....Goodnight air", he would point to the stars, and then tap the top of his head.  It took me a few times to realize he thought I was saying,  "Goodnight hair", since he had no concept of what "air" was.  

And so, the tradition continues (of course, we started actually saying, "Goodnight hair" after that).  I don't really want to clue him in on the truth just yet.   The day I have to explain it will be a pretty sad day for me.