Take a bottle of Diet Coke. Open. Add Mentos. Duck.

If you haven’t tried it, you’ve probably seen a video of it. If you haven’t seen a video — or, if you just want to watch it again, here you go:







Big fountains — they make them larger in the video by re-capping the bottles and allowing the foam to shoot out of a pin-sized hole in the lid. But why do the Diet Coke and Mentos “react” in the first place?

A team lead by Dr. Tonya Coffey, a professor of physics at Appalachian State University in Boone, North Carolina, set out to determine just that. Their findings?

First, it’s not an acid-base reaction akin to what happens when one mixes vinegar and baking soda. Coffey’s team noticed that the pH of the resulting stream of Diet Coke hadn’t changed, ruling that out. In fact, it’s most likely not a chemical reaction at all, but rather, the disruption of water molecules by a rough object with high surface area. Coffey explained to New Scientist:

“Water molecules like to be next to other water molecules, so basically anything that you drop into the soda that disrupts the network of water molecules can act as a growth site for bubbles. And if you have rough candy with a high ratio of surface area to volume, then there’s more places for the bubbles to go.”

And when a lot of bubbles get together, there’s no where to go but up. Way up.

Other key factors? The surface tension of the liquid, for one. The lower the surface tension, the more incredible the geyzer. Aspartame — the sweetner in Diet Coke — lowers surface tension more than the sugar found in regular Coke, making the diet variety more effective. This also plays a role in the choice of the mint: Mentos have a coating made of gum arabic, which further lowers the surface tension of the soda when it is introduced. Second, the density of the mint matters: Mentos fall quickly to the bottom of the bottle, collecting bubbles on the way down. Less dense mints which float or bob do not have as dramatic an effect. (So don’t bother mixing in a pack of Tic Tacs.)

But other elements have no noticeable effect. Caffeine-free Diet Coke, for example, created as an effective fountain as its more common, caffeinated, sibling.

Bonus fact : Since 2000, the Coca-Cola Company has introduced nine new varieties of Diet Coke. Of the nine, only six were introduced in the United States — Diet Raspberry Coke, Coca-Cola Light Sango (blood orange flavored), and Diet Coke with Citrus Zest were not offered for retail sale in domestic markets.

From the Archives: Cola Enforcement Agency: About the cocaine inside your Coke.

Related: Diet Coke and Mentos.