A shell that cracks during hard cooking makes a mess and a sulfurous stink, while a shell that doesn’t peel away cleanly makes an ugly, pockmarked egg. A traditional preventative measure for both problems is to poke a pinhole in the wide end of the shell, but studies have found that this doesn’t make much difference. The best way to avoid cracking is to heat fresh eggs gently, without the turbulence of boiling water. On the other hand, the best guarantee of easy peeling is to use old eggs! Difficult peeling is characteristic of fresh eggs with a relatively low albumen pH, which somehow causes the albumen to adhere to the inner shell membrane more strongly than it coheres to itself. At the pH typical after several days of refrigeration, around 9.2, the shell peels easily. If you end up with a carton of very fresh eggs and need to cook them right away, you can add a half teaspoon of baking soda to a quart of water to make the cooking water alkaline (though this intensifies the sulfury flavor). It also helps to cook fresh eggs somewhat longer to make the white more cohesive, and to allow the white to firm up in the refrigerator before peeling.