All these methods have one thing in common: They rely on applying just enough force to the center of the egg — the egg equator, if you will — to crack the shell without shattering it. And science has finally come up with an explanation for what comes naturally to most home cooks: Eggs crack best around their equators, says MIT mechanical engineer Pedro Reis, because of their geometry. He and a young colleague, Arnaud Lazarus, have just published a paper in Physical Review Letters demonstrating a link between an eggshell's geometry (it belongs to a class of shapes known as ovoids) and a mechanical property called rigidity — the quality that, along with strength, determines how much force an object can withstand before it cracks.
Scientifically, strength and rigidity are distinct, though related, concepts: The shape of an eggshell can determine its rigidity without affecting its strength. Reis explains the distinction by pointing out the difference between two eggshells, one of which is unbroken, and the other of which has tiny cracks in the shell. The two eggshells have the same rigidity, but the latter has less strength, because it breaks under less applied force than the egg with no cracks.
Reis' and Lazarus' model is a mathematical formula that predicts what will happen when you poke thin, shell-like structures at specific points. That is essentially what it takes to crack an egg: a sharp, targeted strike at the specific point where the egg's structure is weakest. The shell has a threshold beyond which it can't absorb any more force, and once that threshold is crossed, the shell will crack.
Reis became interested in studying eggshells after successfully walking on a few cartons of eggs without breaking them, a popular party trick among the geekerati. "I didn't believe it until the first time I did it, but you can actually stand on a carton of eggs," he said. The key is to align the eggs so that the narrow pole is pointing upward, and step in such a way to distribute your weight over the entire surface area, to avoid overloading any one eggshell. (It helps to go barefoot, Reis says.) Usually, it takes a little more than 5 ½ pounds of force to crack an eggshell — much less than the weight of a human being — but the precise amount of force needed depends on the direction in which that force is applied and how much the force is distributed (or not) over the surface of the shell.