In addition to protein, vitamins, and minerals, milk contains nitrogen, oxygen, and carbon dioxide. When heated, the concentration of carbon dioxide tends to increase, which causes the appearance of small air bubbles that run towards the surface of the container. Now, the proteins dissolved in the milk prevent these bubbles from increasing in size, by covering them with a resistant and elastic film, the so-called layer of cream.
When boiling, the bubbles collide with each other until the film that imprisons them is violently broken. This causes the milk to pour out.
