you have to write the sonnet

"Can't she see what's going to happen?" Calvin asked.

"Oh, not in this kind of thing." Mrs. Whatsit sounded surprised at his question. "If we knew ahead of time what was going to happen we'd be—we'd be like the people on Camazotz, with no lives of our own, with everything all planned and done for us. How can I explain it to you? Oh, I know. In your language you have a form of poetry called the sonnet." [...]

"You mean you're comparing our lives to a sonnet? A strict form, but freedom within it?"

"Yes." Mrs. Whatsit said. "You're given the form, but you have to write the sonnet yourself. What you say is completely up to you." Madeleine L'Engle A Wrinkle in Time