Stay-at-home science project: Use fire to push an egg into a bottle

Welcome to PopSci’s at-home science projects series. On weekdays at noon, we’ll be posting new projects that use ingredients you can buy at the grocery store. Show us how it went by tagging your project on social media using #popsciprojects.

If you like fire and you like eggs, this is the project for you. Simply put, we’re going to toss a few lit matches into an empty glass bottle, plug the top with a peeled hard-boiled egg, and watch mayhem unfold. When you’re done, all (or at least some) of the egg will be in the bottle, thanks to the power of the Earth’s atmosphere.

This experiment clocks in at around half an hour, but you’ll spend most of that time preparing the eggs. The actual egg-bottling lasts less than five minutes. So if you want to multitask and boil some other eggs to eat, we won’t blame you. Just don’t get fancy with the egg you’re going to use for this. You’ll want one that’s fully hard-boiled—not soft-boiled or anything with a runny yolk.

The bottle is important too. Glass is required because fire can damage plastic, and its mouth should be slightly smaller than the diameter of an egg. A perfectly sized mouth makes it more likely the egg will enter the bottle whole. The smaller the mouth, the more likely it is that the egg will be destroyed in the process, and if it’s too small, it may not work at all.

And before we start, a warning: Matches are not playthings, so make sure you or another adult does all the fire-making. Don’t toss the burned-out matches straight into the trash, either—dunk them in some water just to be safe.

Stats

• Time: 25-30 minutes
• Difficulty: easy

What you’ll need:

• At least one egg
• A glass bottle
• Matches (or small birthday candles)
• A metal pot with a cover
• Water

Instructions

1. Boil the egg. If you’ve ever boiled eggs before, you know how to do this. If not, here’s a quick breakdown.

2. Peel the egg. You’ll want to ensure the egg is as smooth as possible after you take off its shell, so be careful here. If the egg’s surface is pitted, it may not slide smoothly into the bottle and could rip apart more easily.

• Tip: If you have a tried-and-true method for peeling eggs, use that. If not, peeling an egg under cold water seems to make things easier. You can also bounce the egg on the counter to break the shell and start peeling from the widest end where the air pocket is.

3. (Optional) Insert a candle into the narrowest end of the egg. If you’re not using candles, you can skip this step.

4. Make fire. Light three matches at once and drop them into the bottle. If you’re using a candle, light it now, making sure you’re holding the egg with the candle upright like you’re about to really disappoint someone on their birthday.

• Note: You can also light a piece of paper on fire and drop it into the bottle, but we found that to be less consistent than matches.

5. Put the egg on the bottle. Once the matches are in the bottle, you’ve got to move fast before the fire goes out. Place the egg on top, narrowest end down. If you’re using a candle, turn the bottle upside down and place it on top of the egg, over the candle. Let the candle burn for a couple seconds, then turn the bottle right-side up.

6. Wait. Once there’s no more fire, watch the egg. You’ll see it begin to slide into the bottle until it falls, whole, onto the charred remains of the matches, or is brutally torn apart by atmospheric forces. Either way, science has happened, and you’re a success.

• Note: If the egg does not enter the bottle, you can try to recover it by pressing lightly against where it’s touching the bottle’s rim. The pressure will equalize (it might sound like a fart) and you should be able to pick it up to start again.

How it works:

The primary reason the egg moves is the pressure difference between the air inside the bottle and the rest of the Earth’s atmosphere. “It doesn’t get sucked into the bottle, it gets pushed into the bottle from the atmosphere outside,” explains Rhett Allain, a physics professor at Southeastern Louisiana University.

If you merely place an egg on top of an empty bottle, it’s just going to sit there. You can call it art, if you want, but it won’t do anything interesting. That’s because the air inside the bottle is pushing up on the egg with the same amount of force that the air outside is pushing down. The two forces cancel out and the egg doesn’t move. Add fire, though, and things change.

Inside the bottle, the lit matches quickly heat the air under the egg. Some of that air will escape through the mouth of your bottle—if you get the egg in place quickly, you may see it move. Once the egg seals the bottle and the matches go out, the air inside begins to cool. That, in turn, lowers the pressure, says Richard Jones, a physics professor at the University of Connecticut.

And because atmospheric pressure at sea level is equal to almost 15 pounds per square inch, Allain says even a small pressure drop inside the bottle is enough to get the egg moving.

Some articles have explained this phenomenon as the result of fire creating a vacuum inside the bottle. That’s not what’s happening, Allain says. If there was a vacuum, there would be no gas in the bottle at all. Instead, there’s just less.

Updated May 11 at 12 p.m.: This story has been updated to more accurately reflect how the egg goes into the bottle.