First, courtesy of food blog thekitchn, a little background on why milk boils over, unlike a pot of plain water:
As milk heats, the water in its structure starts evaporating from the surface. This concentrates the remaining fat and proteins into a thicker layer at the top of the pot. This layer eventually becomes so thick that water vapour rising through the milk can't break through very easily and gets trapped.
While this complex interaction of protein and fat is what makes milk awesome for things like the creamy froth that goes into cappuccinos, it's also the reason that it boils over. The quick and dirty fix for the problem is to put a long-handled spoon into the pot as it is heating. The spoon breaks the surface tension and the skim of milk fat and protein forming and allows the steam to escape without violently erupting.
