During the festival, shops are closed to leave the streets open for the ghosts. In the middle of each street stands an altar of incense with fresh fruit and sacrifices displayed on it. Behind the altar, monks will sing songs that it is believed only the ghosts can understand. This rite is called "shi ge'r", meaning "singing ghost songs". Fifteen days after the feast, to make sure all the hungry ghosts find their way back to hell, people float lanterns on water and set them outside their houses. These lanterns are made by setting a lotus flower-shaped lantern on a piece of board. Hungry ghosts are believed to have found their way back when the lanterns go out. It is believed that the soul contains elements of both yin and yang. The yin is the "kui", or demon part, and the yang is the "shen", or spirit part. When death occurs, the "kui" should return to earth, and the "shen" to the grave or family shrine. If a ghost is neglected, it will become a "kui". The "shen", or ancestral spirit watches over its descendants, and can bring good fortune if properly worshipped. Hungry ghosts are different from the ghosts of Chinese and Vietnamese traditions, which all people are believed to become after death. According to the Nyāyānusāriṇī, there are three main groups of hungry ghosts, each of which are divided into three sub-groups: Sixteen hungry ghosts are said to live in hell or in a region of hell. Unlike other hell dwellers, they can leave hell and wander. They look through garbage and human waste on the outskirts of human cities. They are said to be invisible during the daylight hours but visible at night. Some hungry ghosts can only eat corpses, or their food is burnt up in their mouths, sometimes they have a big belly and a neck as thin as a needle (this image is the basic one for hungry ghosts in Asian Buddhism). According to the "Saddharmasmṛtyupasthāna Sūtra", there are thirty-six different types of hungry ghost. There are many folk beliefs and taboos surrounding the Hungry Ghost Festival. Spirits are thought to be dangerous, and can take many forms, including snakes, moths, birds, foxes, wolves, and tigers. Some can even use the guise of a beautiful man or woman to seduce and possess. One story refers to a ghost which takes the form of a pretty girl and seduces a young man until a priest intervenes and sends the spirit back to hell.