Last week a buddy of mine rang me for advice. He was staring into a carton of eggs that had been in the back of his fridge for weeks, and there was no expiration date on the carton. “Kim!” he exclaimed, “how do I know if they’ve gone bad?!” The answer is simpler than you’d think. You don’t have to cook them, crack them, smell them, or shake them to find out if they’re still fresh. Nope, the answer is much, much simpler than that.

To test your egg’s freshness, place it in a pot of cold water. That’s it.

The fresher the egg, the more it will stay submerged, sunk inside the water, holding flat against the bottom of the pot. The less fresh an egg, the more it will swim to the top.

This advice first came to us by 18^th century food writer Hannah Glass, and it remains just as true for home cooks today. But it seems too simple to be true. Is it just an old wives’ tale?

Actually, it turns out there is scientific truth behind the egg-in-water theory. An egg’s shell is ever so slightly porous, meaning that it lets small amounts of air pass through it. Once a fresh egg pops out of old Henny, it begins to slowly release water and carbon dioxide from inside the shell as small bits of air come in. Over weeks, the inside of the egg loses a bit of mass while gaining a bit of buoyant-providing air, and you are left with an egg that floats.

Here’s the basic rundown for how your eggs should look in cold water:

• If just one to two days old, the egg will lay almost flat along the bottom of the pan.
• When an egg is up to one week old, it will start to angle upwards a bit, but will still stay touching the bottom of the pan.
• After about two weeks, the egg will start to stand upright, with most of the tip of the egg still touching the bottom of the pan.
• If the eggs is a few weeks old, it will have a bit of bounce to it, bobbing a bit off of the bottom of a pan. The egg isn’t necessarily rotten at this point, but you’d be served well to serve it hard-boiled or baked pretty well.
• Beyond about four to five weeks after the egg is laid, it will float completely to the top of the water. This is what you might call a “rotten egg.” Here’s a slew of unofficial suggestions from Recycle This for using those old eggs.

Sadly, I didn’t have this keen bit of information handy for my friend when he was testing out his eggs. My advice at the time: “If they’re less than a month old, they should be fine.”… not the worst advice, but it sure would have been backed up with the ol’ floating water trick.

Let us know if you’ve tried the egg-in-water trick for testing your own eggs at home.

Image adapted from Steve A Johnson, Flickr, Creative Commons 2.0