“Hard-boiled egg” is a misnomer—you should never actually boil an egg at all, for two reasons. One, the boiling action of the water tosses eggs around the pan, causing them to crash into each other and possibly split open. If they crack open right away, the white seeps out of the shell and that spells disaster. The second reason is that the protein in the egg whites firms up, or coagulates, at a much lower temperature than you might expect—around 140 to 190 degrees F. (Water boils at 212 degrees F.) Although the yolk may be cooked perfectly, the whites have spent too much time in the heat and turn rubbery. They should cook at the barest simmer (or 180-190°F) to avoid these issues.