 The gun-type and implosion-type designs were codenamed "Thin Man" and "Fat Man" respectively.These code names were created by Robert Serber, a former student of Oppenheimer's who worked on the Manhattan Project.He chose them based on their design shapes; the Thin Man was a very long device, and the name came from the Dashiell Hammett detective novel "The Thin Man" and series of movies.