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Why are some boiled eggs hard to peel?

Few kitchen tasks are more tedious (and annoying) than struggling with hard-to-peel boiled eggs. Why is it that sometimes the shells slip off so effortlessly and other times, you end up with super clingy shells that come off in uneven pieces along with chunks of egg white? It turns out, the answer is pretty simple: the fresher the eggs, the more difficult they are to peel smoothly. It comes down to the pH level of the eggs, which gradually rises as they age.

Fresh eggs have low acidity, which causes the egg white to stick tightly to the inner shell membrane. But eggs are naturally porous, so as time goes on, they'll release some humidity and carbon dioxide, while increasing the air in their shells. The more air there is between the inner membrane and the shell, the easier the eggs will be to peel after you've boiled them. The bottom line? If you're planning to make hard-boiled eggs, make sure you use ones that are between 7 to 10 days old.