Because after small animals are born, just like babies, everything is taken care of by their parents. Their parents usually identify their children by smell. Once a small animal is touched, its body odor will change, and its parents may not recognize it, so it is difficult for the small animal to get food and parental protection. Therefore, people can only touch kittens when they can open their eyes, or when puppies are able to find their mothers to nurse after two weeks of birth.