The best way to boil an egg, according to science

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Eggs that are 7 to 10 days old are ideal to make hard-boiled eggs that will peel easily, says Shirley Corriher, author of CookWise: The Hows and Whys of Successful Cooking. Researchers discovered that older eggs are more alkaline, which makes it easier to remove their shells.

Egg cartons are marked with a Julian date—a numeric system that goes from 001 for Jan. 1 through to 365 for Dec. 31—indicating which day the eggs were packed, which is often the same day they were laid. But if you don’t feel like counting the day of the year to figure out how old your eggs are, keep in mind that eggs are typically delivered to supermarkets between 3 and 5 days after they’re laid. For smooth peeling, then, simply remove any airtight seals from the packaging to allow the CO2 to escape and refrigerate the eggs in their carton for 4 to 7 days before hardboiling.

But you’re not always home free if you buy your eggs from the grocery store. About 10 percent of commercially produced eggs are sprayed with an odorless, tasteless mineral oil to replace the cuticle. The mineral oil seals the pores in the eggshell and prevents CO2 from escaping, which means that oiled eggs will never peel well.

Below, you can see what a really bad egg looks like in comparison to really fresh one. The one on the left is most likely 3 or more months old (from when it was laid, not the date you actually bought them).

The reason this method works is because the eggshells are porous, which means they allow some air to get through. Fresh eggs have less air in them, so they sink to the bottom. But older eggs have had more time for the air to penetrate the shells, so they're more buoyant and will float.

Adding about a teaspoon of baking soda to the cooking water increases the alkalinity, which will make the eggs easier to peel later on.

If you're making deviled eggs or some other recipe where you need to have the yolks of your hard-boiled eggs centered, you may want to take an extra step. Wrap a rubber band around a carton of eggs and store it on its side or pointy side down overnight and your yolks will be centered.

A cracked egg is a sad hard-boiled egg: usually the egg white leaks out and makes the whole shebang harder to peel properly once it's done.

You can warm eggs in two ways: take the eggs out of your fridge and let them sit until they're at room temperature, or put them in hot tap water for up to five minutes. An extreme change in temperature is the most common reason why eggs crack during the cooking process, which is why you want them at room temperature and to cook them at a gentle simmer.

Some swear by pricking the bottom of the shell (otherwise known as the fat end) with a thumbtack to prevent cracking. Chemistry professor Arthur Grosser explains that every egg has a pocket of air inside (as we've shown you in our guide to telling when eggs are really expired), and pricking the shell allows the air to escape. Otherwise as the egg cooks, the air is forced against the fragile shell, causing it to crack.

Others argue that pricking the shell makes the structure weaker and more likely to crack under the pressures of cooking. It's up to you. This is where cooking gets fun—experimentation.