When you mix Diet Coke and Mentos candies together, an explosion results. Physicist Tonya Coffee published a paper in the American Journal of Physics explaining why this happens. New Scientist summarizes Coffee's findings:
Instead, the vigour of the jets depends on various factors that affect the growth rate of carbon dioxide bubbles.
The rough, dimply surfaces of Mentos encourage bubble growth because they efficiently disrupt the polar attractions between water molecules, creating bubble growth sites. […]
"Water molecules like to be next to other water molecules, so basically anything that you drop into the soda that disrupts the network of water molecules can act as a growth site for bubbles," Coffey told New Scientist. "And if you have rough candy with a high ratio of surface area to volume, then there's more places for the bubbles to go."
Low surface tension also helps bubbles grow quickly. Measurements showed that the surface tension in water containing the sweetener aspartame is lower than in sugary water, explaining why Diet Coke creates more dramatic fountains than sugary Coke.
The phenomenon scales nicely. With enough Mentos and Diet Coke, you can build a rocket car. Or, like Harley Morenstein of Epic Meal Time, you can wear a full-body suit made of Mentos candy, sit over a dunk tank filled with Diet Coke, and go for a swim.