Why is Ms. Garcia uncomfortable with her lesson about the Twin Towers in the novel, Towers Falling?

Miss Garcia seems nervous when she introduces the Twin Towers as the first lesson's theme. By the end of the story, the reader knows that Miss Garcia is nervous because she witnessed firsthand the towers falling as she looked out her fifth grade classroom window. She was so shocked by the attacks that she had nightmares for a long time. Knowing that people were helping each other at that difficult time gave her faith in humanity again.