Waterfalls often appear to resist freezing because of the kinetic energy and constant motion of the falling water. For water to turn to ice, its molecules must slow down enough to form rigid bonds, a process that is difficult when the water is being churned and aerated as it plunges. The sheer velocity of the water creates friction and internal heat, which, while small, can be enough to keep the water slightly above the freezing point. Additionally, waterfalls are often fed by large bodies of water or underground springs that have significant thermal mass, meaning they lose heat slowly. However, in cases of extreme, sustained cold, waterfalls can freeze, but they do so in a unique way: "frazil ice" (tiny crystals) forms in the turbulent water and begins to stick to the rocks, eventually building up "ice dams" or a frozen exterior shell. This creates the illusion of a "frozen waterfall" while the liquid water often continues to flow hidden inside the icy column, protected from the frigid air.