way up to entire cities. Themes Old Man's War sits in the military science fiction genre but themes of the ethics of life extension, friendship, marriage, the significance of mortality, what makes one human, and individual identity are present within the novel. Aging plays its biggest role near the beginning of the novel with the CDF being able to find a way to reverse the effects of aging. The themes of marriage and friendship are explored in the characterization of John Perry through his continued love of his dead wife and his later meetings with Jane. When it comes to identity and humanity John Perry is the focus of attention and his characteristics by the end of the novel determine whether or not the reader believes John Perry is still human. Scalzi states that he was influenced by Robert Heinlein's Starship Troopers and modeled his book's format after Heinlein’s novels. He wanted to make the story as sympathetic as possible to the reader in which one can understand events such as being in a war. Scalzi took what he learned about Heinlein and produced four lessons on how to create a novel centered on characters. These lessons are that a story should only exist for its characters, make room in the characters for the reader, make the characters talk like people, and make the characters act like people. His novel's themes were based on the four lessons in which to make a character as connectable as possible while still keeping his theme of space military. Sequels The Ghost Brigades, the sequel to Old Man's War. The Last Colony, the third book in the series. Zoe's Tale, the fourth book in the series. The Human Division, the fifth book in the series. The End of All Things, the sixth book in the series. The Sagan Diary, a novella taking place between The Ghost Brigades and The Last Colony. The audio version available from the author's website. After the Coup, a short story featuring Harry Wilson. Questions for a Soldier, a short story featuring John Perry. Adaptations Netflix announced plans in 2017 to make a film based in the universe of Old Man's War. Previous failed attempts to release adaptations of Old Man's War include a film project by Paramount Pictures and a TV adaptation by Syfy. Appearances in other works A character can be seen reading the book in an episode