What ingredent makes the mentos and Diet Coke explode?

The reaction between Mentos and Diet Coke is caused by the presence of gum arabic in the candy. Gum arabic is surface-active compound, meaning it causes surface tension to be lowered. This property allows the carbon dioxide gas in the soda to form bubbles more readily, which then rapidly rise to the surface and cause the eruption. Other mints and candies made with gelatin or sugar do not have this same surface-active property and therefore do not produce the same reaction.