Why milk often boils over and spills? Because when it heats, the water in its structure starts evaporating from the surface.
As a result, the remaining fat and proteins concentrate into a thicker layer at the top of the pot, which eventually becomes so thick that water vapor rising through the milk can’t break through very easily.
Instead, it gets trapped, causing it to explode violently.
To avoid this, simply put a long-handled wooden spoon into the saucepan as it heats. The spoon acts to break the surface tension — allowing the steam underneath to escape smoothly without an eruption.