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I was going back through the app that gave us the Wendy question, when I found this:

Eggs sink in water when they are fresh and float when expired

I also found this advice:

Fresh eggs will sink to the bottom of the bowl and lie on their sides. Slightly older eggs (about one week) will lie on the bottom but bob slightly. If the egg balances on its smallest end, with the large end reaching for the sky, it's probably around three weeks old. Eggs that float at the surface are bad and should not be consumed. source

Is any of this validated by scientific evidence, or is it just an old wives' tale?

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Salt or fresh water? –  Brian M. Hunt Jun 2 '11 at 19:57
    
@Brian I'm not sure, no one specified. I'm assuming fresh, but obviously the salt concentration of the water would make a difference with regard to buoyancy of an object. –  Monkey Tuesday Jun 2 '11 at 19:58
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Anecdotal: I just put an egg in my sink that expired 25 days ago. It didn't float, but balanced on its little end. –  dan04 Jun 4 '11 at 3:04
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1 Answer

up vote 10 down vote accepted

An egg absorbs air through the shell and discard water/vapor. Old eggs will have enough air to float.

As the egg ages, moisture and carbon dioxide leave through the pores of the shell, air enters to replace them and the air cell becomes larger.

An air cell (space) forms when the contents of the egg cool and contract after the egg is laid. The air cell usually rests between the outer and inner membranes at the egg’s larger end.

See this page for more info.

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In other words, when eggs float it is an indicator that they've lost moisture. This is not a reliable indicator for whether the egg is safe to eat or not. Storing the egg in a drier environment will cause them to start floating earlier and vice versa. –  Agrajag Dec 18 '13 at 9:37
    
@Agrajag: Reliable, no. But it helps to set reasonable expectations before cracking it open. :) –  Macke Dec 18 '13 at 12:46
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