Pop Rocks candy works very similarly to the carbonation in soda pop. The candy has tiny bubbles of pressurized carbon dioxide gas trapped inside. When the candy starts dissolving in the saliva in your mouth (or pretty much any other watery liquid), the gas escapes. So it's not really a chemical reaction as much as a physical release of small amounts of trapped gas. Sometimes you still hear people say that eating Pop Rocks with soda would make your stomach explode, but as they showed on Mythbusters, you'd have to swallow several pounds of Pop Rocks all at once for this to be possible.