Children imitate others to fit in and sometimes ostracize those who do not fit in. Margot does not have the history that the other children share and so she does not immediately fit in with the others.
"Margot stood apart from them, from these children who could never remember a time when there wasn't rain and rain and rain..."
Rather than imitate to fit in however, Margot shares her knowledge of Earth, a place the others have never lived, and of the sun that shines regularly there. On Venus, where the others were born and where Margot has only recently moved, the rain falls continuously and the sun shines only once every seven years.
The children feel hostility toward Margot for her experiences on Earth with the sun that they so long for and cannot remember ever seeing. When she shares what she has experienced and knows of the sun, typical of many children who feel a sense of jealousy or threat from the unknown, some of them confront her, insisting she does not know about the sun. The author conceptualizes this with the boy who tells her that nothing is going to happen on the day the sun is supposed to finally come out.
Feeling the need to fit in, others join in this group mentality and ostracize her, finding comfort in their shared hostility towards her. This text offers two examples of why bullying occurs: fear of the unknown and jealousy.