[The following is an exact transcript of this podcast.]

Physicists study all kinds of curious things, from the missing matter in the universe to the strange behavior of electrons. But none of these is quite as curious, or dramatic, as what happens when you drop a Mentos into a bottle of Diet Coke.

You’ve probably run across the YouTube videos. A person, often wearing a lab coat and goggles, plops a mint into a bottle and, whoosh, a foamy jet of sticky soda sprays 20 feet into the air. Now physicists from North Carolina think they know why. They repeated the reaction using a variety of catalysts, from the classic Mentos to table salt, as well as a handful of solvents, including caffeine-free Coke and seltzer.

They found that what’s going on is physics, not chemistry. The rougher the stuff that gets dropped in the fluid, and the faster it sinks, the more spectacular the eruption. Microscopic nooks and crannies encourage the growth of carbon dioxide bubbles. That carbonation fuels the geyser. And things that sink quick create lots of bubbles that seed even more bubbles as they rise. The explosive results appear in the June issue of the American Journal of Physics. It’s not rocket science--unless the bottle’s upside down.

—Karen Hopkin