Egg whites cannot be whipped to their maximum volume if they come in contact with any fat beyond the trace amount they already contain. Even a speck of yolk will cause a problem because one-third of yolk's composition is fat. So separate your eggs diligently, and if a speck of yolk happens to taint the egg whites, remove that yellow villain with one of the sharp, jagged edges of an empty eggshell. Yolk, however, is not the only potential source of volume-reducing fat. Grease in the whipping bowl or on the whipping instrument is just as bad.