The float test does not determine whether an egg has gone bad, but it does provide a useful indication of the age of an egg. To perform the float test, place the egg in a large bowl of water. If the egg sinks or stays at the bottom, it is still fresh. An older egg will either stand on its end or float.

Why does a rotten egg sink?

As more of the egg decomposes, more of its mass is converted to gases. A gas bubble forms inside the egg so an older egg floats on its end. However, eggs are porous, so some of the gas escapes through the eggshell and is lost to the atmosphere.

Is it safe to eat eggs that don't float?

If the eggs sink to the bottom and lay flat on their side, they're still fresh. However, if they sink, but stand on one end at the bottom of the glass or bowl, they're not as fresh but still edible. Of course, if any eggs float to the top, they shouldn't be eaten.