Fire and Ice
As we noted in "Form and Meter," the sound of the poem is structured around the repetition of the syllables "ire" and "ice." The poem swings back and forth between these rhymes to mimic the speaker's choice between them.
Thematically, the turning point of the poem occurs in line 5, but, sonically (based on the sound), it happens in the last two lines, when a single eight-syllable line is broken half in order to double up on the rhymes. The last two lines are like the punch line of a joke: the "ba-doom-CHING!" if you will. They create big pauses, as if the speaker is holding his breath to build anticipation. The payoff of the poem is the dramatic understatement of the word "suffice," a word that sounds almost comically non-threatening, until you remember we're talking about the end of the world.
The use of the word "suffice" is a neat trick for another reason, too: it captures the two patterns of alliteration of the poem. The beginning of the poem is marked by repeated "s" sounds, and line 4 contains two words that beginning with "f." Just think of how different the poem would be without those last two lines!