I should have realized...

but in my denseness didn't . . . that Nemerov's riddle is rhymed iambic pentameter:

What is this creature who must sweat all day
In the mill of making, in the field of fight,
And then, instead of sleeping, spend all night
To counterfeit the paper for its pay?

So much for my poetic ear that I didn't hear the poetry. In my defense, I note that nobody else on the Milton list mentioned this until Louis Schwartz pointed it out.

Now, as anyone can see . . . and it's odd that I had to see it before I could hear it . . . the rhyme scheme is abba, the rhythm is generally unstressed-stressed, and the feet are trippingly five.

I suppose that one could debate about the rhythm and therefore the feet.

Does having the form correct aid toward solving the riddle? I don't know, but poetry makes more pleasant the process of thinking the riddle through.

Incidentally, Nemerov's lines either are or are from his poem A Riddle the Sphinx Forgot, published in volume 120 (July 1972) of Poetry.