In the historical context of Number the Stars, physical descriptors that would be entirely trivial in any other context are profoundly important to the two girls's respective understanding of the world that they occupy. Annmarie, who is a young Christian, has never had cause to believe that her good friend Ellen is any different than her because of her faith or physical appearance. However, that all begins to change when the Nazi occupation begins seizing Jewish households and families in Copenhagen. Annmarie soon realizes that her fair complexion, her faith, and her family history is one that gives her a great deal of leverage when dealing with German soldiers. They do not seem to have any intention of capturing or even extensively detaining her, and treat her with more respect than they ever would a family like Ellen's.

Ellen, on the other hand, is the subject of much scrutiny as she has a hair and eye color that is decidedly Jewish. Much of the time, Annmarie is able to pass Ellen off as her sister. Even with an incredibly convincing photograph, the Nazi's remain skeptical. Their deep sense of nationalism channels their leader's lust for genocide.

This attention to physical characteristics is part of what makes the book so terribly effective in portraying the farcical nature of war. By demonstrating a child's struggle to wrap her head around the absurd realities of war, such as the fact that people with certain physical traits need be more careful, Lowry is able to display the struggle from a cognitive distance. It is painted in a far more absurd light.