Groucho Marx once said: "Outside a dog, a book is man's best friend. Inside a dog, it's too dark to read." According to a new book on humor by two cognitive scientists, this joke is funny because our brain anticipated something other than the image of a someone trying to read a book literally inside a dog. "The brain constantly generates presumptions about what will happen next. It calculates where a pedestrian will go, what a speaker will say, how a banana you’re peeling will look under the skin. In short, the brain 'produc[es] real-time anticipation on all important topics.'" The punch lines of a joke twists the image our brains produce.