Why do M&Ms make Pepsi fizz?

[Hokienautic]

Okay, this is probably a dumb question, but it aroused my curiosity. I was at lunch and had a bottle of Pepsi and some M&Ms. Boredom overcame me, and I dumped a pair of M&Ms in the Pepsi ... first it made a pretty big bubble, then started fizzing pretty steadily, for a good five minutes. It appears that the Pepsi ate the shell off the M&Ms, and that's what caused the fizzing, but ... WHY? Anything else that fizzes in Pepsi. I know supposedly Coke can clean old pennies. Is this similar?

[CalMeacham]

I'll have to try this out at home. Until then, one possible explanation is that the surfaces of the M&M act as nucleation sites for the carbonation bubbles in the soda. Try dumping salt into your Pepsi (or beer) and you should get impressive fizzing. This isn't a chemical reaction, but the Pepsi ought to dissolve the shell eventually, just as water will.

Just a quick off-the-cuff answer. I'll have to check it out. Pepsi IS acidic, but it's not clear to me that acis should react with that sugar coating.

[Duck Duck Goose]

"Nucleation sites" sounds good to me. If you sprinkle a teaspoon of sugar into any diet soda, you get a huge fizzy effect.

[Skwerl]

One time I was dying for a Coke, and all we had was Diet...Being 10 years old, I figured the only difference was the sugar content, so I ingeniously dropped a spoon of sugar in. The soda fizzed out of the can and all over the counter and floor. Then I drank some water.