When the water fills this container, its weight causes it to tilt, spilling the water and thus operating the mill. A water mill typically works throughout the day, grinding grains without the need for human intervention. To prevent water from entering the grinding section, a long rod is often spread out in a conical shape and covered with materials such as thatch, bark, straw, or reed to form the mill's cover. During winter, when the water level is low and freezes, the water mill cannot be used. However, in mountainous villages where wild animals are a frequent threat, metal cans are hung on the axis at night to make loud noises and scare off the animals. In Jeollanam-do, this device is sometimes called a "thunder mill."