How does the human digestive system actually work? Why can competitive eaters eat so much, so fast? Why do we like certain food textures better than others? Science writer Mary Roach is the author of books including “Spook: Science Tackles the Afterlife" and "Stiff: The Curious Lives of Human Cadavers." She answers these questions and more in her latest book, “Gulp: Adventures on the Alimentary Canal."