Some of the Whys of Eggs, Milk and Cheese

Why do the whites of eggs beat up more lightly than the yolks? Why does a black ring some· times form around the yolk of a hard cooked egg? What causes the film on the top of heated milk? Why is cheese sometimes ropy and tough when cooked? And on and on-dozens of questions about every day occurrences usually explained by, "It does because it does." The true explanations are simple and interesting in the light they throw upon cooking processes.