Why re-boiling milk doesn't boil over again? When I heat milk, at some point it begins to boil over.
When I reheat the same milk after it has reached boiling point once before, it doesn't boil over again, it just boils &quot;normally&quot;. Why is this?
I haven't done extensive testing, and I can't seem to find information about this anywhere; be it in English or in my native language. But here is what has happened to me:

I make a homemade hot beverage with low fat milk, cocoa, sugar and coffee (a pseudo-Mocha). Enough for 2 cups.
I boil the pseudo-Mocha, and hope to pay enough attention to prevent it boiling over, but there is still the &quot;rising bubble&quot; phenomenon occurring.
I pour out one cup, the rest of the liquid stays in the pan.
A few hours later, I boil the pseudo-Mocha again.
This time, no bubbly phenomenon occurs, just &quot;classical&quot; boiling.
If you heat food high enough, you can cause chemical changes.  Letting it cool back off doesn't necessarily reverse these changes.
Typical changes include:

deactivation of enzymes
denaturing (breaking down) proteins
converting sugars into more complex forms
evaporation of part of the liquid

In general, we call this 'cooking', but there's specifically a term in English for that you're doing to the milk: scalding
I suspect that you've denatured the proteins in the milk, and proteins can create foams (such as what happens when you whip egg whites or the liquid from canned beans), but there might be other processes that have happened as well.
I also know that scalding isn't called for typically in modern cookbooks.  Part of this is because today's milk is homogenized and pasteurized, and so some of the chemical changes that we get from scalding have already happened.  But there are still quite a few people who insist that you need to scald milk for certain types of bread making, or you won't get the proper rise and final texture from the loaves.