The poem opens by focusing on a little boy—referred to as "you"—who has just begun to cry. At this point, it's not yet clear what has happened to make the boy's expression twist into an image of pain and anger.
Instead of clarifying the circumstances of the little boy's exasperation, though, the first four lines ("Your mouth ... bright eyes") simply draw attention to the nature of the boy's unhappiness. They create a stark juxtaposition between the boy's previous joy and his current mood by highlighting the quick transformation of "laughter" into tears and "howls." In turn, the poem encourages readers to experience the raw and overpowering onslaught of emotion that the little boy himself is feeling in this moment.
To achieve this effect, the unidentified speaker uses anaphora, repeating the word "your" at the beginning of each clause. This not only calls attention to the little boy (who is the primary subject of the poem), but also spotlights the fact that readers should inhabit the boy's perspective as if they are the ones whose laughter has suddenly transformed into "howls." Accordingly, the world of this little boy ultimately emerges as relatable and immediate, allowing readers to remember what it's like to be young and deeply upset.
On that note, it becomes apparent that the boy's discontent is all-consuming, since his entire body —which was previously "relaxed"—has gone stiff and rigid in "three-year-old frustration." This description suggests that the boy is unable to keep whatever is upsetting him at bay, thereby indicating that he doesn't yet have the coping mechanisms to process troubling emotions. In this sense, the boy's innocence comes to the forefront of the poem, framing him as a youngster who hasn't yet developed the ability to work through hardship.
The consonance in this section also adds to the general sensation of what it's like to be overwhelmed, as the guttural /r/ and tapping /t/ sounds repeat rather relentlessly:
Your mouth contorting in brief spite and hurt,
your laughter metamorphosed into howls,
your frame so recently relaxed now tight
with three-year-old frustration
These sounds creating a low growling, biting sound that is reminiscent of the child's frustration. Consequently, readers feel the overpowering quality of the boy's emotions, which are intense and seemingly impossible to control.