Eggs lose water as they age. This results in more air inside the eggshell, so an older egg will float higher in a bowl of water than a fresh egg. The USDA Food Safety and Inspection Service recommends using eggs within 3 to 5 weeks of buying them, but if you forget when you bought a carton of eggs and the carton has no expiration or sell by date, you can test the freshness of an egg with a bowl of water.
Fill a bowl with tap water to at least an inch above the height of an egg.
Place the egg gently into the bottom of the bowl full of water.
Keep any egg that lies flat or at an angle at the bottom; these eggs are still fresh. Eggs that stand straight up on the bottom of the bowl are older, but still safe to eat; use them in baked goods or for boiled eggs.
Discard any egg that floats to the top of the water, as it has most likely gone bad.
Discard any egg that has an unpleasant odour when you crack it open, regardless of whether it passes the float test.