Here's one of the world's great mysteries: why do cats scratch on the back door to get you to open it, and then when you do, they just stand in the doorway and do nothing? What are they thinking? What is going on there? Scientists say, we might never know, because cats are the most uncooperative research subjects ever. David Grimm, Deputy News Editor at Science, is the author of the new book Citizen Canine: Our Evolving Relationship with Cats and Dogs.