Answer: Nucleation

Dropping Mentos candies into a 2 liter bottle of Diet Coke and watching the massive geyser of soda erupt from the bottle is, and has been for over twenty years, a staple of flashy science class demonstrations beloved by students and teachers alike.

It would be easy to assume that what you’re witnessing—a huge eruption of extremely foamy cola that rockets six feet or more into the air—is a strong chemical reaction, but it actually isn’t. The primary force driving the reaction is physical and, on a very dramatic level, is the same physical reaction that causes streams of bubbles in champagne.

Carbon dioxide, when in solution, wants to attach to things. You see this when you pour a glass of that aforementioned champagne, for example, and bubbles begin to form at the bottom of the champagne flute and slowly stream upwards. The microscopic imperfections at the bottom of the glass are known as nucleation sites because the gas in the liquid nucleates upon them. You can even form your own nucleation sites by dipping your finger into a carbonated liquid—bubbles will immediately begin forming on the tiny ridges of your skin.

Mentos candies yield such a spectacular reaction due to two primary factors. First, although the candies appear smooth to us, they are—on a microscopic level—covered in thousands upon thousands of tiny holes which make for more than an abundant number of nucleation sites. Second, the candy is heavy and it sinks rapidly to the bottom of the bottle. When you drop the Mentos candy into the bottle, it rapidly pulls the carbon dioxide gas out of the solution and, thanks to the rapid descent, a column of gas is created from the top to the bottom, quickly forcing the soda out of the bottle.

Many people mistakenly attribute the effect to some “special” element in Diet Coke itself, like the artificial sweetener (the aspartame in diet drinks does lower the surface tension and causes bigger reactions than with sugared drinks). The reason science teachers use Diet Coke, however, has nothing to do with Diet Coke possessing special properties and everything to do with them not having to deal with liters of sticky sugar-loaded cola all over the school yard or lab.

Even if you knew that nucleation was the source of the reaction, here’s a bit of bonus trivia so you won’t leave empty handed: Mentos weren’t the original candies used in the experiment. The experiment originated in the 1980s when science teachers started dropping rolls of Wint-O-Green Life Savers, threaded together with a pipe cleaner, into bottles of soda. In the late 1990s, the manufacturer changed the size of the Life Savers slightly and they were then too big to fit through the bottle’s mouth and neck. The teachers tried other candies and found that Mentos had the same effect (and were easy to drop into the bottle) and the rest is backyard-science history.