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Why Do Diet Coke and Mentos React?

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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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Big Questions
Why Do Cats Freak Out After Pooping?
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Cats often exhibit some very peculiar behavior, from getting into deadly combat situations with their own tail to pouncing on unsuspecting humans. Among their most curious habits: running from their litter box like a greyhound after moving their bowels. Are they running from their own fecal matter? Has waste elimination prompted a sense of euphoria?

Experts—if anyone is said to qualify as an expert in post-poop moods—aren’t exactly sure, but they’ve presented a number of entertaining theories. From a biological standpoint, some animal behaviorists suspect that a cat bolting after a deposit might stem from fears that a predator could track them based on the smell of their waste. But researchers are quick to note that they haven’t observed cats run from their BMs in the wild.

Biology also has a little bit to do with another theory, which postulates that cats used to getting their rear ends licked by their mother after defecating as kittens are showing off their independence by sprinting away, their butts having taken on self-cleaning properties in adulthood.

Not convinced? You might find another idea more plausible: Both humans and cats have a vagus nerve running from their brain stem. In both species, the nerve can be stimulated by defecation, leading to a pleasurable sensation and what some have labeled “poo-phoria,” or post-poop elation. In running, the cat may simply be working off excess energy brought on by stimulation of the nerve.

Less interesting is the notion that notoriously hygienic cats may simply want to shake off excess litter or fecal matter by running a 100-meter dash, or that a digestive problem has led to some discomfort they’re attempting to flee from. The fact is, so little research has been done in the field of pooping cat mania that there’s no universally accepted answer. Like so much of what makes cats tick, a definitive motivation will have to remain a mystery.

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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Big Questions
Why Do Baseball Managers Wear Uniforms?
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Basketball and hockey coaches wear business suits on the sidelines. Football coaches wear team-branded shirts and jackets and often ill-fitting pleated khakis. Why are baseball managers the only guys who wear the same outfit as their players?

According to John Thorn, the official historian of Major League Baseball since 2011, it goes back to the earliest days of the game. Back then, the person known as the manager was the business manager: the guy who kept the books in order and the road trips on schedule. Meanwhile, the guy we call the manager today, the one who arranges the roster and decides when to pull a pitcher, was known as the captain. In addition to managing the team on the field, he was usually also on the team as a player. For many years, the “manager” wore a player’s uniform simply because he was a player. There were also a few captains who didn’t play for the team and stuck to making decisions in the dugout, and they usually wore suits.

With the passing of time, it became less common for the captain to play, and on most teams they took on strictly managerial roles. Instead of suits proliferating throughout America’s dugouts, though, non-playing captains largely hung on to the tradition of wearing a player's uniform. By the early to mid 20th century, wearing the uniform was the norm for managers, with a few notable exceptions. The Philadelphia Athletics’s Connie Mack and the Brooklyn Dodgers’s Burt Shotton continued to wear suits and ties to games long after it fell out of favor (though Shotton sometimes liked to layer a team jacket on top of his street clothes). Once those two retired, it’s been uniforms as far as the eye can see.

The adherence to the uniform among managers in the second half of the 20th century leads some people to think that MLB mandates it, but a look through the official major league rules [PDF] doesn’t turn up much on a manager’s dress. Rule 1.11(a) (1) says that “All players on a team shall wear uniforms identical in color, trim and style, and all players’ uniforms shall include minimal six-inch numbers on their backs" and rule 2.00 states that a coach is a "team member in uniform appointed by the manager to perform such duties as the manager may designate, such as but not limited to acting as base coach."

While Rule 2.00 gives a rundown of the manager’s role and some rules that apply to them, it doesn’t specify that they’re uniformed. Further down, Rule 3.15 says that "No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club." Again, nothing about the managers being uniformed.

All that said, Rule 2.00 defines the bench or dugout as “the seating facilities reserved for players, substitutes and other team members in uniform when they are not actively engaged on the playing field," and makes no exceptions for managers or anyone else. While the managers’ duds are never addressed anywhere else, this definition does seem to necessitate, in a roundabout way, that managers wear a uniform—at least if they want to have access to the dugout. And, really, where else would they sit?

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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