Fizzy candy is an example of a simple acid-base reaction. A commonly used example of this is baking soda and vinegar. In fizzy candy, the components cannot react unless they are in solution, and for that your saliva works great! Once you put the candy in your mouth, saliva dissolves the sugar coating and then provides a solution for the acidic and basic compounds to react. I have included a website that goes into more detail and also provides a really easy recipe to make your own fizzy candy.

The chemical reaction that causes fizziness is not that different from when you add vinegar to baking soda.It is an acid-base chemical reaction that releases carbon dioxide (CO₂). Most often, the reaction involves baking soda (NaHCO₃) and citric acid. The reaction can only take place when the ingredients are mixed in solution (dissolved in your mouth), which is why they don't react until you eat them.

Thanks for your great question! Don't get any cavities while conducting any experiments!