A final surprise, or a different way of looking at the subject is built into the structure of the sonnet, which generally turns to an alternative perspective either between the octave and the sestet, or in the final couplet. Shakespeare's sonnet 18 turns at both points. It seems as though it is going to be a commentary on the brevity of life, which, like a summer's day, is beautiful, intense, and painfully short. However, Shakespeare turns after the octave to say that the addressee's "eternal summer shall not fade," then explains why this is in the final couplet: this poem will preserve his beauty forever. A poem that seemed to be about the transience of life turns out to be about the permanence of art.
Celia Warren's parody of this sonnet begins by asking whether she should compare a worm to "a bit of string." The poet then goes on to celebrate the "eternal stringiness" of the worm, which has a better claim to immortality even than a Shakespeare sonnet, since worms are ancient creatures, predating humans by hundreds of millions of years.* Though the individual worm looks as insignificant as a bit of string, the resilience of the species is remarkable, making worms a sign of everlasting life.
*It is difficult to say exactly how long worms have existed, since their boneless bodies are not well preserved in the fossil record. However, they appear to have been on earth in some form for at least 500 million years.