Door opening is common, I had several cats do it. One cat learned to vomit in the sandbox. We didn't train him to do it, he somehow understood the general vibe

Door opening is common, I had several cats do it. One cat learned to vomit in the sandbox. We didn't train him to do it, he somehow understood the general vibe that we don't like him vomiting and decided to go to his sandbox to do it (and picked up on the positive feedback from that). I wish we knew how to make them do that because he wasn't the only vomiting cat.