My parents have three cats and they are allowed to roam outside in the backyard and the gardens in the neighbourhood during the day. But in the evening when it dawns we like to have them inside where it's warm and safe. Especially in winter it's important that they're in before it gets too cold. However, one cannot watch the door the whole day and look whether they want in or not; and a cat flap would allow them to bring all kinds of mice and other pests inside. That's the story how it came to this little project.