This is false. A floating egg is older, not necessarily spoiled.
Why Do Some Eggs Float?
After it is laid, the moisture in an egg slowly evaporates by seeping through the tiny pores in the shell. This moisture is replaced by air from the outside. As a result, the small air pocket in the egg enlarges and the egg becomes more buoyant. Eventually it becomes buoyant enough to float. This has nothing to do with spoilage, only with age. Eggs can spoil, of course, but that is not the cause of floating.
Dealing With Floating Eggs
If an egg sinks and lays flat, it is relatively fresh If it sinks but stands upright it means it's a bit older because the air sac has enlarged a bit. If it actually floats to the surface you may want to use a little caution just in case. Break it into a bowl and give it a sniff test before adding to your recipe. An egg that is actually spoiled has an unmistakable foul odor—it smells like, well, rotton eggs! There's no point in wasting a perfectly good egg just because it's a bit long in the tooth.
Some Other "Eggy" Myths for You!
Trouble peeling your boiled eggs? Read this.
Do your eggs crack when boiled? This post may help.
