I tilted my head at him as an invitation to query so of course he did. :)
"Mom, what does that mean.. only the back of him active?"
So I showed him this video.
And he was "OH!"
The simplicity allows the reader to consider the actions of the dog and how simple they are and grin in remembrance (at least it does me).
When I asked what other words she could have used, or even what words words rhymed my lad thought of ground hogs...and how a dog could be sniffing after them, even as he said "that wouldn't fit in the poem though mom" and "it wouldn't be as funny if it didn't rhyme".
I think he has a point don't you?