The most common is anal disease, such as anal fissure, hemorrhoids, this kind of pain, fear of defecation makes the stool shape thin. 2, is the rectal disease, rectal tumor, rectal cancer, so that the shape of the stool changes, some people may be more careful, found not only thin, and irregular stool shape. Normal defecation is usually cylindrical, this time the shape of the stool may occur or one side of the shape change. Rectal tumors or rectal polyps, some of which are very close to the anus, can also cause a change in the shape of the stool. The role of the colon is to absorb water and make the stool dry easily. In general, shape changes are less common.