I read somewhere that old eggs float not because they’re spoiled, but because as they lose moisture over time air takes the place and voila, the eggs float!
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It has to do with the air pocket in the egg. If it floats completely, you probably don't want to be eating it.
See? Another article I read said that even if the egg floats, it may still not be spoiled. The article insisted that the only way to tell if an egg is good is to break it open and smell it!
I do the float test and it's never done me wrong!
The standard is that if it sinks and lays on it's side it's fresh; if it sinks and stands on end, it's less fresh, if it floats, still less. This is due to the size of the air pocket inside; you can also ( with experience) get a good idea by shaking the egg. None of this means the egg is "bad" as in growing salmonella or other bacteria-it's a pretty sterile environment; that can best be judged by smell, but an older egg will be of less flavor and quality, and the white will not whip as well.