Eggshells look solid, but they're actually slightly porous, so the developing chick inside can breathe. This quality can be exploited when dyeing easter eggs:
Often, of course, people do not want the eggs to “sweat”, as the emerging fluid pushes the dye to the side (much like putting salt crystals on a wet watercolor moves that pigment around). The way to avoid this is to make sure the egg and the room are same temperature.
I quite like the effect, particularly when it shows something of how the egg developed, as with this blue egg: the pores seem to be more active around the middle.
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