In Ray Bradbury's short story "All Summer in a Day," the metaphor "I think the sun is a flower" was written in a poem about the sun by the protagonist Margo. In her poem, Margo aimed to describe the glow of the sun as a blooming flower. Though she doesn't specify what kind of flower, the reader might visualize a round golden daisy or poppy. The image of the flower helps capture the round, yellow image of the sun.
In addition, Margo's metaphor parallels with the other children's experience in seeing the sun for the first time. As they look outside as the sun comes out, they see the "great jungle that covered Venus" transform. Suddenly, because of the sunshine, the jungle looks alive, flowing, and full of color, similar to a flower, as we see in the narrator's following description:
It was a nest of octopi, clustering up great arms of fleshlike weed, wavering, flowering in this brief spring.
Hence, as we can see, Margo's metaphor serves the purpose of likening the sun to a flower to capture the sun's color, shape, and glowing warmth. Margo's description of the sun parallels with the effect the sun has on nature found on Venus.