Cats can use pet doors: For families that maintain indoor-outdoor cats, nothing’s more convenient than a pet door. Know who came up with the idea? Gravity King Sir Isaac Newton! According to various tales passed down over the years, Newton was experimenting with light in a darkened room and one of his most prized cats (reportedly named “Spithead”) kept opening the door and disrupting the experiment. To keep his work intact and allow the cat the freedom to roam, he cut a small hole in the door and covered it with cloth. That evolved into what is today known as a pet door, suitable for both cats and dogs. Source: MNN