Why does putting Mentos into a bottle of diet Pepsi cause an explosion?

Who's asking: James Lamb, Virginia Beach, VA

Today's deliberately silly post is willful denial of many bad things that have happened in the last couple of days, and are happening even as I type this. In a rotten world -- a world where leaders are cowards, doctors are terrorists, good people get sick and Beverly Sills is dead -- you can still put a handful of Mentos into a bottle of soda and make magic. Do try this at home. And get well soon, Mary.

Okay, you're asking, why is this Independence Day-related? Because it is a great American tradition to blow stuff up on the 4th of July, and this is a safe way to do it, with no risk of anyone losing a finger or anything catching fire.

If you haven't seen it, go watch the phenomenon here. (The video's a little long, but it's funny and makes an important point: you must use original Mentos mints, the white ones. Mentos gum doesn't work.)

Anyway, how it works is that you open a two-liter bottle of Diet Pepsi, and with a paper funnel, pour in a handful of Mentos mints (I have had the best results with five). Step back and watch the soda shoot into the sky in a spectacular fountain. Theoretically, this will work with any type of soda -- but I've never tried it with sugared soda, because it would get too sticky, and Diet Coke did not produce as big a geyser as Diet Pepsi.

So what makes this happen? Theories abound, but no one knows for sure. In fact, scientists don't agree on whether it's a physical reaction or a chemical one. The theory that makes the most sense to me (an educated layperson) is that the surface of original Mentos are covered with microscopic pits that form "nucleation sites" for carbon dioxide to form bubbles. Dropping Mentos into the soda creates so many bubbles that the gas forces the liquid out of the bottle, and it all goes straight up. (The liquid left in the bottle is flat.) The phenomenon works with bottles smaller than two liters, but the two-liter bottle seems to provide the right amount of air to maximize pressure within the bottle.

James asks the obvious follow-up question, which is whether eating Mentos and drinking Diet Pepsi will cause a similar eruption in your own stomach. The answer to that is no. You chew Mentos before you swallow them, which breaks up the surface tension (getting rid of those nucleation sites) and mixes the Mentos with your own digestive acids. The acids in your stomach break down the carbon dioxide in the soda, too, so there's never an opportunity for the chemical or physical reaction to occur.

That said, I wouldn't pop a handful of Mentos and then swig a Diet Pepsi. I just wouldn't.