You can feel the fizz the moment you open a can of Coke. That fizziness is carbon dioxide gas — CO₂ — dissolved into the liquid under pressure. The can is like a coiled spring: there's a huge amount of CO₂ in there, just waiting for a chance to escape.

The escape needs a starting point. A bubble can't simply appear in the middle of a liquid — it needs something tiny to grip onto. Scientists call these nucleation sites — microscopic rough spots where dissolved gas can break free and form a bubble.

Here's the Mentos secret. Each sweet looks smooth, but zoom in and the surface is covered in thousands of tiny pits — more like a golf ball than a marble. Every one of those pits is a nucleation site: a launching pad for a bubble. The moment a Mentos drops into the Coke, all those launching pads fire at once. Thousands of bubbles form simultaneously, race upward, and carry a column of liquid with them. That's the geyser.

Here's the important part: this is not a chemical reaction. Nothing new is being created. The Mentos is simply unlocking the CO₂ that was already in the drink — all at once — by giving it thousands of places to start. You can see the same CO₂ gas doing very different work in [Lava Lubes], where bubbles lift oil droplets to the surface, and in [The Invisible Fire Extinguisher], where CO₂ is used to smother a flame. Same gas — completely different science.