Conjunction Mal-Function

For the fifth and final time, I'm almost through with this book.  (Note the tattered edges)   As with several other school items this year, we're dragging it on through July, but that's not to say we aren't having a good time with it.
Gus is working his way through the phonics and the stories, but the best part of the book so far (to him) has been the conjunctions.

Yes, conjunctions.

Well, one conjunction in particular...

every time he reads the word "but" to me, he goes into paroxysms of laughter, falling to the floor pointing to his own conjunction and repeating the word "but".  I haven't had the heart to tell him about homonyms yet...