To test the freshness of an egg, fill a pitcher or tall glass full of water and place the egg in the water. If the egg sinks, it is fresh.
If the egg floats, it is old. Egg shells are porous, which allows air to enter the egg. Fresh eggs have smaller air cells than older eggs, so they sink. Older eggs have larger air cells, so they float. However, a floating egg is not necessarily a bad egg. Before throwing away an old egg, crack it open. If it smells and looks normal, it is still usable. If it is discolored or has a foul odor, throw it away.