Milk is a fine suspension of fat and protein globules in a watery liquid containing milk sugar (lactose) and other dissolved substances. Normally the fat is dispersed uniformly in the form of fine droplets and hence remains as an emulsion. But when milk is heated these fine droplets tend to raise up and at temperature of about form a thick layer of cream on the surface. When milk comes to boiling, bubbles of steam formed at the bottom of the vessel rise up and force the thick creamy layer upwards causing the milk to spill.