At the “soft peak” stage, when glossy foam edges retain some shape but droop, and when the foam doesn’t yet cling to the bowl, the somewhat coarse bubbles are still lubricated by plenty of liquid, which would quickly drain to the bottom of the bowl. At the “stiff peak” stage, where the foam is still glossy but now retains a well-defined edge and clings to the bowl, the foam is approaching 90% air, and the egg liquid has been spread so thin that the protein webs in adjacent bubble walls begin to catch on each other and on the bowl surface. There’s just enough lubrication left for the foam to be creamy and easily mixed with other ingredients. This stage, or perhaps just before it, is the optimum for making mousses, soufflés, sponge cakes, and similar dishes that involve mixing and further rising in the oven. Further beating gains little additional volume.