How can you tell if an egg in the refrigerator is fresh? Just toss it into a glass of water. Eggshells are porous, which means that over time, air builds up inside the eggshell. The older they are, the more air builds up and eventually it goes rotten. If the egg is no longer fresh, the excess air will force the egg to float. If it's still fresh, it will sink. Then it's safe and good to eat.