To start, it should be noted that it’s not just Diet Coke and Mentos that “react”; other carbonated beverages will also readily respond to the addition of Mentos. The very high level gist of what’s going on here between the Mentos and the Diet Coke is that ingredients in the Mentos/Coke and, more importantly, the structure of the Mentos allows carbon dioxide bubbles to form extremely rapidly; when this happens fast enough, you get a nice Diet Coke fountain.