Food for thought: Why does a skin form over hot milk?
As heat is applied to the milk, the proteins casein and beta-lactoglobulin start to coagulate, and form a skin on the surface. After further heating, the skin dries because of evaporation, and forms a still firmer barrier. Steam produced under the skin builds up and causes the milk to boil over.
The action of whisking stops the skin from forming (by creating a foam over the surface) and the milk from boiling over. Skin will also form on milky drinks after heating, but adding toppings - marshmallows or whipped cream, say - prevents this.
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