Why does milk overflow when boiled but water does not?

Milk overflows when boiled for the first time because it contains entrapped air. The air expands on heating and tends to escape lifting the surface of milk. Once all the air is expelled, the boiling would be regular and smooth. A second boiling would not lead to overflow. Entrapped air in mother's milk is the cause of pain for babies that goes under the name 'colic pain'. Breast feeding is therefore to be done in small doses at a time.