Sentence Diagramming Question: what to do with WHY

[Ritsumei]

We're using Rod and Staff, and it includes sections on sentence diagramming. We've been playing around with it, messing with some sentences from Harry Potter and other such games. Today, my 13yo stumped me with this sentence:

"Why are there no horses?"

I've looked all over, and I can't figure out what I'm supposed to do with the word "why". If you drop that out of the sentence, the diagram is a piece of cake. But I have no idea what to do with the sentence when it includes the why, and I've searched the internet, but can't find any of the sites that teach diagramming that deal with why, though all the rest of the question words seem much more straight forward.

Thanks!

[HomeAgain]

I had to step back and determine what word "why" modifies. I thought it was "are", which led me to a search to see if it was an adverb.  Consensus is: yes, definitely adverb. https://www.macmillandictionary.com/us/dictionary/american/why_1
It can be other things, but in this case I'd call it an adverb and diagram that way.

[Violet Crown]

We study grammar structurally, so the first question would be "What phrase would replace 'why' in a declarative sentence?" Since that would be a subordinate clause ("There are no horses because this is downtown Manhattan"), you would presumably diagram it the same way you would diagram a subordinate clause. This is more obvious if you reorder the sentence as "There are no horses why?"