Milk skin refers to a sticky film of protein that forms on top of milk and milk-containing liquids (such as hot chocolate and some soups). It is caused by the denaturation of proteins such as beta-lactoglobulin (whey protein). Contrary to some sources, casein, which is the main constituent in cow's milk, is not thermolabile (does not react to heat) and is stable at as high as 100 °C.
When milk is boiled, soluble milk proteins are denatured and then coagulate with milk's fat and form a sticky film across the top of the liquid, which then dries by evaporation.
In Nepal this skin is referred to as turr and many people enjoy consuming the skin along with the milk.