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Why Mentos and Diet Coke Create Fountains

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Rough surface: One of the reasons why Mentos, magnified here, are fountain-makers.

What really makes Mentos and Diet Coke form those fizzy fountains we love to watch on YouTube? Until recently, explanations were mere conjecture. But a physicist at Appalachian State University in North Carolina set her students on the trail. What they found:

What's happening, said the physicist, Tonya Coffey, is that Mentos are an ideal shape, texture, and weight to encourage carbon dioxide bubbles to form in the Diet Coke bottles.

"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," Coffey told New Scientist. "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."

The aspartame in Diet Coke also lowers the surface tension of water, helping create bubble growth sites.

What's more, "Mentos are also fairly dense and sink rapidly, quickly creating bubbles that seed further bubbles as they rise."

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