Need to tell if your eggs are still good to eat? Here’s a quick kitchen test: Fill a deep bowl with cold water and place an egg inside. If it sinks to the bottom you’re good to go. If it bobs on its side in the middle of the bowl, use it today. And if it floats to the top, chuck it: you likely have a rotten egg. While this test can’t guarantee an egg’s freshness with exact certainty, it’s based on sound science.
Eggshells are designed to protect developing embryos by repelling most external substances, but the shell is still somewhat porous and will absorb air over time. The older an egg, the more air it absorbs, and with enough time an air bubble will form inside the egg, which allows it to float. With exposure to outside air comes exposure to bacteria which, along with the natural process of decomposition in what’s essentially a menstrually-expelled gamete, means an egg you don’t want to eat.