As is true for the setting of heated eggs and custards, the key to the stable egg foam is the tendency of the proteins to unfold and bond to each other when they’re subjected to physical stress. In a foam this creates a kind of reinforcement for the bubble walls, the culinary equivalent of quick-setting cement. Whipping exerts two kinds of physical stress on the proteins. First, as we force the whisk through the white, the whisk wires drag some of the liquid with them, and create a pulling force that unfolds the compacted protein molecules. And second, because water and air are very different physical environments, the simple mixing of air into the whites creates an imbalance of forces that also tugs the proteins out of their usual folded shape. All these unfolded proteins (mainly the globulins and ovotransferrin) tend to gather where air and water meet, with their water-loving portions immersed in the liquid and their water-avoiding portions projecting into the air. Thus disturbed and concentrated, they readily form bonds with each other. So a continuous, solid network of proteins pervades the bubble walls, holding both water and air in place.