The Mentos-and-soda geyser effect revealed a few months ago has now spawned a contest on YouTube. Eggheads that we are, we figured that to win we'd have to first understand the mechanisms of the explosion -- so we tracked down a science-fair style explanation:
At first glance, it might seem as if a chemical reaction is taking place. Perhaps the Mentos are alkaline and the soda (despite its name) is acidic. That would make the whole thing a variation on my all-time favorite science bit: the baking soda and vinegar volcano. But a quick read of Mentos ingredients reveals precious little alkali... The water molecules in the soda attract each other strongly but equally throughout the Diet Coke when undisturbed by frisky Mentos. This attraction enables the soda makers to pump extra carbon dioxide into the liquid and keeps all the gas from bubbling out immediately because it takes even more extra energy to push the water molecules apart. Dropping a Mentos (or, as in the demonstrations above, a slew of Mentos) into the soda breaks the surface tension in two ways. First, the candy is uneven on a microscopic level. These tiny bumps and pits make it easier for bubbles to form because the surface tension is suddenly spread out over a wider area. ... The makeup of the Mentos is also helping here. Gum arabic--an oily gum derived from the acacia tree--and the coconut oil contribute to the effect by paradoxically making the water molecules even more strongly attracted to one another than usual. Remember, oil and water don't mix.
If you don't have any Mentos handy, you can always enter our stupid-sports contest instead; you've got 27.5 hours left.