There are a few cases in which you should not cook an egg, regardless of how recently you bought it. If the shell is cracked at all, discard the egg because it is much more likely to be contaminated with salmonella than an egg with an intact shell. A strong odor from an egg or significant discoloration could also be a sign of a bad egg. In addition, if the shell is noticeably dirty, discard the egg or wash it before using. A egg that comes in contact with dirt on the outside of the shell can become contaminated.

Eggs will stay fresh for much longer than one week after purchase if you store them properly. Although many refrigerators have space in the door for eggs, this is one of the worst places to store them. Every time you open the fridge, this spot warms up. Instead, store your eggs on one of the regular shelves, ideally toward the bottom or back of the fridge, where it stays coldest.

If you do not know how old an egg is, apply a few tests. Look at the date stamped on the carton. If this is a "best by" or "use by" date, the eggs should still be safe to eat before this date if you stored them in the refrigerator the whole time. You will usually have at least three weeks between the date when you buy the eggs and the date stamped on the carton. Another method to test freshness is to put a raw egg in a bowl of water. If it sinks, it is fresh. If it floats, it is old and might be rotten. Test it by cracking it into a glass bowl, looking at it and smelling it for anything unusual.

Some methods of preparing eggs, such as over-easy or sunny-side-up, are best when the eggs are extra fresh because the yolk stays more compact and is less likely to break. To cook older eggs, scramble them or use them in baking. Wait until eggs are a week old to hard-boil them; the membrane thickens with age, making them easier to peel.