The ball behind the kitten's ass is its anus. The anus is a part of the animal's body and a very important part. Besides defecation, anus can also excrete urine, liquid and gas. Kitten's anus is much smaller than human's, so it needs less excrement. In order to ensure the health of kittens, owners need to ensure that they have enough water and high-quality diet to promote normal excretion. In addition, it is necessary to regularly check the excrement of kittens to ensure that its color and quality are the same as normal.