Zotz candies bubble when you bite into their super-sour center. How come?
A look at the ingredient list reveals the answer. Besides sugar and corn syrup, colors, and flavors, the candy contains malic acid, sodium bicarbonate, and tartaric acid. In other words, it's just an acid-baking soda reaction.
Here's a video of Zotz in water.
If you can't view the video, here's my six-year-old's illustration of the process. It's almost as good as the video (says his mother).
Next time you try Zotz, remember to enjoy the bubbles. You're eating a self-contained acid test!