In Chapter One, why was Mrs. Murry upset at Mrs. Whatsit?

After finishing her sandwich, Mrs. Whatsit gets up to leave. While she is putting on her boots she looks at Mrs. Murry and tells here that “there is such a thing as a tesseract.” Mrs. Murry is shocked, since she hadn't said anything. To herself, Mrs. Murry wonders, “How could she have known?”