I had some eggs that were going to expire, so I decided to boil them before they went bad. When I put them in the water, however, I noticed they were were standing big-end up. Weird!

Had they gone bad? I checked Google and learned they were still edible. According to this article on Why do eggs float?:

Old eggs float in fresh cold water because of a large air cell that forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger and this acts as a buoyancy aid.

Sounded good to me, but as another reader pointed out, this explanation isn’t quite right:

You are partially correct, however, it is not the fact that air is getting into the egg that makes it float. It is the fact that mass is leaving the egg.

So the moral of this tale is don’t eat eggs which are completely floating and remember density determines whether something will float.