[–] watch_listed 0 points 8 points (+8|-0) ago 

The protein in the milk reinforces the bubbles and allows then to stack up on one another, while the bubbles in water simply pop. I am not a scientist, but that's my basic understanding.

[–] circumstantial [S] 0 points 1 points (+1|-0) ago 

[–] Lurking_Commenter 0 points 1 points (+1|-0) ago 

Denaturation probably plays a vital role in restructuring these proteins here when heat is involved.

[–] SimonWest 0 points 2 points (+2|-0) ago 

nice question, it wasn't til I read it that I'd thought about it or knew i wanted to know the answer

[–] circumstantial [S] 0 points 0 points (+0|-0) ago 

Thanks. Probably because I boil milk a lot so it crosses my mind. Glad you enjoyed.

[–] Bricolagefantasy1 0 points 0 points (+0|-0) ago 

In general, because milk is really a complex mixture, not a simple pure liquid like water. A lot is going on while milk is boiling. They dont behave linearly.