Two decades ago as a young teenager I was intrigued when I dropped a single M&M into a 2 liter of Sprite and saw it quickly overflow. After some brief thought, I figured the CO2 previously dissolved into the soda was being rapidly displaced by the sugar on the coating of the M&M, since sugar is much preferred to go into solution in water compared with CO2.
These guys at eepybird.com are taking that same soda experiment to a new level, creating a small-scale Bellagio fountain. And they seem to think the cause is something called nucleation sites, or a propensity for bubbles to form on the rough surface of the candy itself. I still think the cause is sugar dissolving into the soda and displacing the CO2, especially since the surface of an M&M is pretty smooth, and the M&M has about the same effect as Mentos. Probably the more intense spray of a diet soda is because much less other stuff is dissolved into a diet drink than a sugared one, leaving room for more CO2. (Takes much less mass of artificial sweetener to equal the taste of sugar.) What do you guys think?