A lot, actually. A reheated hard-boiled egg will explode when you cut or bite into it. That’s something I learned the hard way.
Instead of that experiment, here’s a trick for reheating or warming a hard-boiled egg, if you’re like me and prefer eggs to be slightly warm rather than ice cold. This comes from Teri Lynn Fisher and Jenny Park, authors of The Perfect Egg.
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To serve hard-boiled eggs warm, “place a few of [the eggs] (with their shells intact) in a heatproof bowl, pour in boiling water to cover, and cover the bowl. Let the eggs stand for about 10 minutes, then remove them from the water, peel, and serve.”