All you need for the floating egg experiment is a glass wide enough to place an egg flat on its side. Fill three quarters of the glass with water and gently leave the egg sink. In just a matter of seconds you can determine whether the egg is good to eat. In addition, you can repeat the experiment as many times as you like with the other eggs in your fridge.
If the egg you have just placed in the water sinks to the bottom and lies flat on its side, it means that it is fresh and safe to consume. If its tip stays on the bottom while its bottom (the broader end) points towards the surface, it means that the egg is still suitable for consumption, but you should use it for baking or hard-cooked eggs recipes. On the contrary, if the egg floats, you must discard it at once, as it is rotten.
According to the University of York, during the egg's aging process, the water inside its shell starts evaporating, replaced by air and other gases (accounting for rotten eggs' unpleasant smell). As the egg gets older, it ends up having more gas inside its shell than water. Gas is lighter that water, so when you place rotten eggs on the glass, they are bound to float.
Apart from the floating egg experiment, you can also check the freshness of eggs, judging by their weight. However, only experienced cooks can be certain about an egg after feeling its weight. Another method is to break the egg onto a flat plate and see it its yolk is round and the white still thick. This method is reliable, but more tedious than the floating egg experiment, as afterwards you must place fresh eggs onto a bowl or pan, while you have to clean rotten eggs off the plate. You may also have to do this several times if you check a series of eggs.