Question

what is the main reason for milk to rise up while boiling?

Answer 1

here you're answer

Before we start, we must be aware of that fact that milk is not a normal liquid like water. Milk is far more complex as it is made-up of 87 percent water, 4 percent proteins and 5 percent lactose. When we boil milk, fat, sugar, proteins and minerals get separated.

As these are lighter than milk, they collect on the surface to form the cretiger

Now, as the milk continues to get heated, out of the 87 percent of water, some of it gets converted into vapour. These bubbles of vapour start to rise above but are stopped and not allowed to escape by the cream which has been formed on the surface.