The test everyone knows
Drop an egg in a glass of cold water. Sinks flat? Use it. Floats? Bin it. Standing upright on the bottom? "Use it soon," they say.
We've all heard this. The good news: it is grounded in actual physics. The less good news: it tells you less than most people assume.
Why it works (up to a point)
Ever wonder why an egg's buoyancy changes at all? A freshly laid egg has almost no air inside — the contents fill the shell nearly completely. As it ages, two things happen simultaneously: moisture slowly evaporates through the porous shell, and the air cell at the wide end grows to fill the gap left behind.
The bigger the air cell, the more buoyant the egg. A week-old egg lies flat on the bottom. A three-week-old egg tilts upright. A very old egg floats because it is mostly air. The test is essentially measuring the size of that internal air pocket.
What floating doesn't tell you
Here's the part the float test misses: freshness and safety are not the same thing.
An egg can float and still be perfectly edible if it was stored in a cool, stable environment. An egg can sink to the bottom and be spoiled if the shell cracked and bacteria entered. The air cell signals age; it says nothing about microbial contamination or whether the yolk has already broken down inside.
The better test
Crack the egg into a small bowl before adding it to your dish. A fresh egg has a firm, domed yolk that sits high, with thick white hugging it tightly. An older egg spreads flat — still usable, but better scrambled than poached.
A genuinely bad egg announces itself the moment the shell breaks. That hydrogen sulphide smell is impossible to mistake for anything else. If you're uncertain, your nose will settle it in under a second.
So should you bother?
Yes — as a quick first pass when you have no idea how old the eggs are and don't want to waste a bowl finding out. Just calibrate your expectations: sinking is reassuring, floating is a warning, not a verdict.
Always crack into a separate bowl before adding to a batter or a pan. One extra second of checking saves the whole dish.
