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Why Do Diet Coke and Mentos React?

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Wikimedia Commons

Combine Diet Coke and Mentos, and the result is explosive—Diet Coke shoots out of the bottle like a miniature, sticky Old Faithful. The reaction is so intense, you can make a rocket propelled by the resulting geyser. But what's the science behind this reaction?

In June 2008, Dr. Tonya Coffey of Appalachian State University and her physics students published a paper on the phenomenon in the American Journal of Physics. They were inspired by a 2006 MythBusters episode that, according to the paper, "did a wonderful job of identifying the basic ingredients in this reaction ... [but] did not sufficiently explain why those ingredients affect the explosion, nor did they provide direct proof of the roughness of the Mentos—a tall order for an hour-long television program." Coffey and her students decided to dig deeper.

It's All About Texture

Coffey and company discovered that the ingredients in the Mentos and Diet Coke and, more importantly, the structure of the Mentos, allow carbon dioxide bubbles to form extremely rapidly. When this happens fast enough, you get a nice Diet Coke fountain. (It’s not just Diet Coke and Mentos that react; other carbonated beverages will also readily respond to the addition of Mentos.)

Each Mentos candy has thousands of small pores on its surface which disrupt the polar attractions between water molecules, creating thousands of ideal nucleation sites for the gas molecules to congregate. In non-science speak, this porous surface creates a lot of bubble growth sites, allowing the carbon dioxide bubbles to rapidly form on the surface of the Mentos. (If you use a smooth surfaced Mentos candy, you won’t get nearly same the reaction.) The buoyancy of the bubbles and their growth will eventually cause the bubbles to leave the nucleation site and rise to the surface of the soda. Bubbles will continue to form on the porous surface and the process will repeat, creating a nice, foamy geyser.

In addition to that, the gum arabic and gelatin ingredients of the Mentos, combined with the potassium benzoate, sugar or (potentially) aspartame in diet sodas, also help in this process. In these cases, the ingredients end up lowering the surface tension of the liquid, allowing for even more rapid bubble growth on the porous surface of the Mentos—higher surface tension would make it a more difficult environment for bubbles to form. (Compounds like gum arabic that lower surface tension are called “surfactants”).

Diet sodas produce a bigger reaction than non-diet sodas because aspartame lowers the surface tension of the liquid much more than sugar or corn syrup will. You can also increase the effect by adding more surfactants to the soda when you add the Mentos, like adding a mixture of dishwasher soap and water.

Size Matters

Another factor that contributes to the size of the geyser is how rapidly the object causing the foaming sinks in the soda. The faster it sinks, the faster the reaction can happen, and a faster reaction creates a bigger geyser; a slower reaction may release the same amount of foam overall, but will also create a much smaller geyser. This is another reason Mentos works so much better than other similar confectioneries: The candies are fairly dense objects and tend to sink rapidly in the soda. If you crush the Mentos, so it doesn’t sink much at all, you won’t get a very dramatic reaction.

The temperature of the soda also factors into geyser size. Gases are less soluble in liquids with a higher temperature, so the warmer your soda is, the bigger your Mentos-induced geyser will be. This is because the gases want to escape the liquid, so when you drop the Mentos in, the reaction happens faster.

What Doesn't Work

While caffeine is often cited as something that will increase the explosive reaction with the soda, this is not actually the case, at least not given the relatively small amount of caffeine found in the typical 2-liter bottle of soda generally used for these sorts of Diet Coke and Mentos reactions.

You’ll also sometimes read that the acidity of the soda is a major factor in the resulting geyser. This is not the case either. In fact, the level of acidity in the Coke before and after the Mentos geyser does not change, negating the possibility of an acid-based reaction—though you can make such an acid-based reaction using baking soda.

Daven Hiskey runs the wildly popular interesting fact website Today I Found Out. To subscribe to his “Daily Knowledge” newsletter, click here.

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holidays
What Are the 12 Days of Christmas?
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Everyone knows to expect a partridge in a pear tree from your true love on the first day of Christmas ... But when is the first day of Christmas?

You'd think that the 12 days of Christmas would lead up to the big day—that's how countdowns work, as any year-end list would illustrate—but in Western Christianity, "Christmas" actually begins on December 25th and ends on January 5th. According to liturgy, the 12 days signify the time in between the birth of Christ and the night before Epiphany, which is the day the Magi visited bearing gifts. This is also called "Twelfth Night." (Epiphany is marked in most Western Christian traditions as happening on January 6th, and in some countries, the 12 days begin on December 26th.)

As for the ubiquitous song, it is said to be French in origin and was first printed in England in 1780. Rumors spread that it was a coded guide for Catholics who had to study their faith in secret in 16th-century England when Catholicism was against the law. According to the Christian Resource Institute, the legend is that "The 'true love' mentioned in the song is not an earthly suitor, but refers to God Himself. The 'me' who receives the presents refers to every baptized person who is part of the Christian Faith. Each of the 'days' represents some aspect of the Christian Faith that was important for children to learn."

In debunking that story, Snopes excerpted a 1998 email that lists what each object in the song supposedly symbolizes:

2 Turtle Doves = the Old and New Testaments
3 French Hens = Faith, Hope and Charity, the Theological Virtues
4 Calling Birds = the Four Gospels and/or the Four Evangelists
5 Golden Rings = the first Five Books of the Old Testament, the "Pentateuch", which gives the history of man's fall from grace.
6 Geese A-laying = the six days of creation
7 Swans A-swimming = the seven gifts of the Holy Spirit, the seven sacraments
8 Maids A-milking = the eight beatitudes
9 Ladies Dancing = the nine Fruits of the Holy Spirit
10 Lords A-leaping = the ten commandments
11 Pipers Piping = the eleven faithful apostles
12 Drummers Drumming = the twelve points of doctrine in the Apostle's Creed

There is pretty much no historical evidence pointing to the song's secret history, although the arguments for the legend are compelling. In all likelihood, the song's "code" was invented retroactively.

Hidden meaning or not, one thing is definitely certain: You have "The Twelve Days of Christmas" stuck in your head right now.

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Big Questions
Where Does the Phrase '… And the Horse You Rode In On' Come From?
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Horses may no longer be the dominant form of transportation in the U.S., but the legacy of our horseback-riding history lives on in language. When telling people off, we still use the phrase “... and the horse you rode in on.” These days, it’s rare for anyone you're telling to go screw themselves to actually be an equestrian, so where did “and the horse you rode in on” come from, anyway?

Well, let’s start with the basics. The phrase is, essentially, an intensifier, one typically appended to the phrase “F*** you.” As the public radio show "A Way With Words" puts it, it’s usually aimed at “someone who’s full of himself and unwelcome to boot.” As co-host and lexicographer Grant Barrett explains, “instead of just insulting you, they want to insult your whole circumstance.”

The phrase can be traced back to at least the 1950s, but it may be even older than that, since, as Barrett notes, plenty of crude language didn’t make it into print in the early 20th century. He suggests that it could have been in wide use even prior to World War II.

In 1998, William Safire of The New York Times tracked down several novels that employed the term, including The Friends of Eddie Coyle (1972) and No Bugles, No Drums (1976). The literary editor of the latter book, Michael Seidman, told Safire that he heard the term growing up in the Bronx just after the Korean War, leading the journalist to peg the origin of the phrase to at least the late 1950s.

The phrase has had some pretty die-hard fans over the years, too. Donald Regan, who was Secretary of the Treasury under Ronald Reagan from 1981 through 1984, worked it into his official Treasury Department portrait. You can see a title along the spine of a book in the background of the painting. It reads: “And the Horse You Rode In On,” apparently one of Regan’s favorite sayings. (The book in the painting didn't refer to a real book, but there have since been a few published that bear similar names, like Clinton strategist James Carville’s book …and the Horse He Rode In On: The People V. Kenneth Starr and Dakota McFadzean’s 2013 book of comics Other Stories And the Horse You Rode In On.)

It seems that even in a world where almost no one rides in on a horse, insulting a man’s steed is a timeless burn.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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