Isaac Newton may have invented the cat door. While studying at Cambridge University, his experiments kept getting interrupted by his cats scratching at the door, so he had the Cambridge carpenter drill two holes in the door to allow cat and kittens access.
Isaac Newton had a reported fondness for dogs and cats, so the story is plausible, according to HowStuffWorks. In addition, one of Newton's contemporaries wrote some years after the scientist's death that there were two holes of about the right sizes for a cat and a kitten in the study door. The smaller door, however, remained unused as the kitten simply followed his mother through the larger door.