In diet Coke®, and all soda, there are a lot of CO₂ molecules trapped in solution. The slow release of CO₂ from solution is what normally makes the calm bubbling in soda. But what if you were to put a catalyst for gas release into a bottle of diet Coke®? That’s exactly what a Mentos^® does. Dissolved gases need a surface to start forming a bubble (let’s just assume that’s true, which it is), and the Mentos^® candy provides a huge amount of surface area because it is a very porous material. So add a Mentos^® and many, many more CO₂ bubbles can form at the same time, which leads to a sugary eruption.