We should start by saying that no mice were harmed in the writing of this poem. No need to dial PETA.
So, what's going on here? The important thing to note about this line is Collins' preposition choice. He doesn't say drop a mouse onto a poem. If that were the case, we would get the image of someone dropping a mouse onto a piece of paper.
The mouse might take a look at the poem on the page, but mice are about the cheddar, something poems cannot provide. Instead, by choosing the preposition "into," Collins gives the poem physical space, a structure.
Your metaphor detector should be flashing and making a bunch of whoooop, whoooop noises. Our little rodent friend wandering inside a poem is functioning as a metaphor for space (not the final frontier kind of space, the form and structure in poetry kind of space). Poems come in many different shapes and sizes, many different forms and structures. Collins wants us to think about the space, the structure of a poem—to think of a poem as something you can get inside of and roam around in.
Sounds fun, right?
and watch him probe his way out,
As long as we've dropped the mouse in there, we might as well see what he does. With line 5 and 6 together, we get a pretty clear image of a mouse in a maze—something most of us have seen in science class or perhaps even felt like on the first day at a new school.
Collins wants us to experience how, as readers, we move into and through a poem. How we get in and how we get out.
Is it a straight shot from the poem's front door to the back, or is it an intricate maze that makes us feel lost, and then hopeful, and then finally free?
And how do we actually move through it anyways? By reading? Sensing? Hardboiled detective work?