Are you staring down a lone egg that's been lurking in your fridge, wondering if it's still fresh? Assuming they're properly handled, eggs should keep for weeks past the sell-by date stamped on the box. That said, all it takes is cracking into one bad egg for you to yearn for extra assurance that it is in fact still good to eat.
For a surefire test, take a glass or small bowl and fill it with water. Gently add the egg. If it sinks, it's fresh; if it floats, toss it out ASAP. Eggshells are porous, so as an egg ages, moisture and carbon dioxide levels slowly reduce inside and are replaced with air, making it less dense. If the egg has lost so much moisture that it's buoyant, it's sure to be spoiled.