A Diet Coke and Mentos eruption (or Diet Coke and Mentos geyser) is a reaction between a carbonated beverage and Mentos candies that causes the beverage to spray out of its container.
The numerous small pores on the candy's surface catalyze the release of carbon dioxide (CO2) gas from the soda, resulting in the rapid expulsion of copious quantities of foam. Although any carbonated beverage will produce a similar effect, the reaction was popularized using Diet Coke for seemingly producing the best results.
Imagine what could happen in the stomach of small children?