Re: What makes pop fizz?

Carbon dioxide is the gas that makes pop fizz, soda bubble, and soft drinks 
effervesce.  Drinks are either carbonated naturally (like mineral water), 
through a fermentation process (like champagne), or artificially by 
dissolving and/or injecting carbon dioxide in the beverage (like pop or 
most beer).  No matter how it gets there, there is usually more gas than 
the liquid can hold (sodas are super-saturated solutions), so when you open 
up the bottle (or can), the gas wants to escape.  The gas molecules usually 
start to congregate at bumps or scratches in the package surface.  The 
molecules quickly merge together and form bubbles that then rise to the 
surface and escape into the air.

So, that's my quick answer to a great question!  Hope it helps.

Kieran