I raised chicken and found this to be true. You can test how "good" an egg is by putting it in a bowl of water. A brand nw egg will sink and barely bounce. As the egg ages,air enters through pores in the shell. At about two weeks of refrigeration, it will bounce and stand on one end. Most store bought eggs areat least two weeks old. You should not use eggs that float outright. As air enters, so can bacteria. This is why hens avoid pooping on eggs. They will sometimes "hold it" for up to three weeks as they age. If you want details, google it. I could explain the phenomenon, but I am not feeling particularly articulate today.