THE TOOLS THAT MADE nuclear war a reality had code names from fiction. A fan of hard-boiled novelist Dashiell Hammett, Manhattan Project physicist Robert Serber dubbed one “Thin Man” for a Hammett protagonist; a second “Fat Man,” from The Maltese Falcon, and a third “Little Boy,” as world-weary Sam Spade calls a hoodlum. Thin Man went unused; B-29 crews dropped Little Boy on Hiroshima and Fat Man on Nagasaki. In Little Boy, 10 and a half feet long, 28 inches wide, and weighing 9,700 pounds, a 6.5-inch cordite gun fired a large piece of uranium-235 into a smaller one to cause an explosion equal to 12,500 tons of TNT. Nine feet long and 59 inches wide, the 10,800-pound Fat Man detonated a dynamite wrap to mash a plutonium core and yield a blast equal to 21,000 tons of TNT.