Explanation:  Mentos + Diet Coke = Nucleation

According to a 2006 episode of Mythbusters, it is believed that the caffeine, potassium benzoate, aspartame (artificial sweetener) and the CO₂ gas in the soda (Diet Coke works best) combine with the gelatin and gum Arabic of the Mentos to create the explosive effect.  Mint Mentos work best because its surface is covered with many small holes, allowing CO₂ bubbles to form quickly and in great quantity (Nucleation).  Mythbusters was also able to replicate the experiment using rock salt instead of Mentos.

Thanks for the question Penelope!