Normally, as steam bubbles emerge from the milk, they quickly burst, well below the pot's rim. However, if a film forms, the milk bubbles are trapped between this skin and milk. As these bubbles grow in number, they push the skin upward until eventually the skin and some of the bubbles may overflow the pot.
The first rule of avoiding a time-consuming cleanup chore is to never turn your back on the pot for even a short duration - the trapped steam can suddenly begin expanding. And don't forget to stir the milk frequently and thoroughly, particularly if you are heating it above 140F, the approximate temperature at which some of the milk protein begins to coagulate. Unless you have a good reason to do otherwise, keep the milk comfortably below the simmer point.
Even better, use a microwave to heat your milk because it reduces the cleanup chore. Milk can easily scorch in a pot set upon the range; even if the burner is set to low, the pot's bottom can reach 300F or higher. Milk is less likely to scorch in a microwave because no portion of the milk gets hotter than the boiling point and the container never gets hotter than the milk. The best container for heating milk in a microwave is a Pyrex-style cup because it lets you monitor the liquid and is easy to transport and clean.
** Asian Recipes **