The major component of milk is water (83-87%). Its other constituents are protein (3.5%), sugar (5%) and fat (4-7.4%). When heated, fat being lighter than water, floats as a creamy layer on the top and water vapour, in the form of steam bubbles, is trapped under it. Further heating results in the formation of more number of bubbles. These bubbles expand and lift the creamy layer causing it to overflow. In the case of water, the steam bubbles break as they reach the surface.