What causes soda to fizz and how can it be stopped? When you go to a soda fountain and pour some soda into your cup, frothy fizz is generated at the top of the cup. Obviously, it has something to do with carbonation. However, when a cup of soda is just sitting and is not being filled or refilled, there is no fizz.  

What causes the fizz? Is the fizz generated by the crashing of the soda into the cup?

I would also like to know how to stop the fizzing produced when the soda cup is being filled/refilled: Can I slow the rate at which the soda falls? Will spinning the liquid on the way down to the cup help attenuate the fizzing? What about changing the material from which the cup is made?
The soda is supersaturated with dissolved carbon dioxide since in the sealed container the soda is under pressure using carbon dioxide. A number of things can cause the formation of nucleation sites which degas the solution by forming bubbles which float to the surface. The foam is due to the surface tension of the liquid solution. The lay understanding is that the soda is "flat" when the soda is no longer supersaturated with carbon dioxide.  

It should be noted that even in a saturated solution most of the carbon dioxide is in solution as a dissolved gas and not carbonic acid. Since soda solutions are acidic anyways, this also favors the dissolved gas as opposed to carbonic acid. 

To minimize fizzing:


Use a clean glass. (Bubbles emanating from some sort on the
glass indicate that the glass is dirty.)
Use wet ice at the freezing temperature of water (not ice from
fridge below 32 F).
Cool the soda before pouring gently onto the side of the glass.
In other words do not just dump it into the center of the glass onto the
ice. Imagine trying to pour water into the glass without creating
any bubbles.