Diet Coke and Mentos is the trick of dropping mint candies into a two-liter bottle of Diet Coke to set off a fast foam eruption that shoots soda several feet into the air1. The reaction runs on nucleation as carbon dioxide in the soda clings to tiny bumps on the candy's shell, forming bubbles fast enough to push liquid out of the bottle. Diet Coke reacts more strongly than regular Coke, an effect chemists credit to aspartame lowering the drink's surface tension.

The trick took hold online in the mid-2000s after a choreographed fountain video pushed it beyond the classroom demos that teachers like Steve Spangler had been running for years2. It became a middle-school experiment, a nightly-news filler segment, and a reference point for early viral video1.

The setup is simple. Open a two-liter bottle of Diet Coke, drop several Mentos mints in at once, and step back before the geyser hits4. Many builders tape a paper tube or a purpose-made geyser tube over the bottle mouth so every candy drops in at the same time, which pushes the foam column higher and keeps hands out of the splash zone.