Like all objects, eggshells have breaking points, or limits beyond which they cannot absorb more force. That limit is lowest where the egg is weakest—its center. That's precisely because the center area is the flattest, says Sinan Keten, a mechanical engineer at Northwestern University. Contrastingly, the top and bottom of an egg are the strongest—and therefore hardest to crack—because they have the most curvature. Think of a structure that is rounded as opposed to flat, like an arched doorway or an arched bridge. The arch is able to hold a heavier load without breaking because it distributes that weight more evenly. This is true for an egg, too. In fact, if you hold an egg between two fingers at each pole and squeeze as hard as you can, it's extremely unlikely you'll be able to apply enough force to crack it. That's because the shell's curves evenly distribute the pressure you're applying.