“Really there’s only one thing to wish for—a magical cure that will allow me to run free outside like a wild animal. But I never make that wish because it’s impossible. It’s like wishing that mermaids and dragons and unicorns were real.” 

This quote reveals how closely Madeline’s supposed illness is tied to her sense of identity. She cannot imagine herself as a healthy teenager and has ruled out the possibility of ever living a normal life. She equates the desire to be healthy with childish fantasies like the imaginary creatures she mentions. The events of the novel eventually overturn this belief and transform Madeline’s sense of self.

“Immediately my mind goes to outer space. I picture a giant mother ship hovering in the skies above us.” 

The image of outer space emphasizes the distance Madeline feels from the outside world due to her confinement inside. Comparing Olly’s family’s arrival to an alien “mother ship” reinforces that feeling of distance and foreignness. The space imagery also foreshadows her childhood dream related in the diary entry of the next section.

“A single breath more, and my life will finally, finally explode.” 

The image of a balloon expanding and then exploding is symbolic of a deep tension in Madeline’s life between her illness and her desire for a normal teenage life. The balloon is white, in keeping with Yoon’s symbolic use of that color, and expresses Madeline’s desire for her repetitive life to be disrupted in some way (to explode). The idea that the balloon can only hold so much air before bursting also reinforces the idea that Madeline is nearing the limit of what she can endure without making a change.