Milk overflows when boiled due to its complex composition, which includes suspended fat globules and casein proteins. Unlike water, which allows steam bubbles to escape easily, milk forms a membrane-like film that traps steam, leading to increased pressure and overflow. This phenomenon is attributed to the separation of constituents in milk upon heating, causing lighter elements like cream to rise. In contrast, boiling water does not create such a film, allowing steam to dissipate without resistance.
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Milk is not a simple liquid — it contains tiny globules of fat and casein which are not dissolved but suspended in water. On heating milk, there occurs a separation of constituents and some elements like cream, being lighter, float up.
After some time, a membrane-like film containing cream and casein is formed. When milk begins to boil, the heat energy is used for the conversion of water into steam.
As a result, the pressure below the film increases suddenly and the rising bubbles of steam makes the milk overflow. On heating water, no such film is formed and the bubbles of steam formed by boiling escape easily without any resistance. Hence, water does not overflow
on boiling.
— Arun Syamal, Mumbai
http://timesofindia.indiatimes.com/...ed-but-water-does-not/articleshow/1973641.cms
Avoid milk or other liquid overflow: When boiling milk or other liquids, put a metal spoon in the pot. The metal deflects heat and will prevent the milk or other liquids from over flowing, saving you from sticky clean ups ...
http://www.sizzlingpots.com/toolbox/tips/168-prevent-milk-overflows.html