At the beginning of the book, Meg recalls that her father, before he went away, had told her that Charles Wallace had a good mind and that Meg should not worry about him. Instead, Charles Wallace, like Meg, just did things at his own pace. Meg thinks of herself as dumb, but her father reassured her that this was not true. When she asked him how he knew that she was not dumb, her father told her that he and his wife, Mrs. Murry, had played games with the children that were really tests of different kinds. Among these "games" were IQ tests. Her father would not tell her what her IQ was, but he said that she and Charles Wallace could do anything that they wanted to, given their high intelligence.

Mrs. Murry knows that Meg and Charles Wallace are very smart because she and Mr. Murry gave them IQ tests before Mr. Murry disappeared. This is important for Meg to know: the local community thinks she and Charles Wallace are "dumb" because they are different. 

Meg tells her father that she doesn't want Charles Wallace to end up "dumb like me." She also wonders, when her father assures her she is smart, if he thinks so just because he loves her. He assures that is not so and that the parents have tested her. She then remembers games her parents played with her that must have been IQ tests.

However, when she asks her father for specifics about what her IQ is, he won't tell her. He does let her know that she and Charles Wallace are smart enough to "do pretty much whatever you like when you grow up." 

Because Charles Wallace has blossomed since her father disappeared, Meg believes what her father said.