What is in sour skittles that makes it explode when put in diet coke?

The reaction between Sour Skittles and Diet Coke is due to the presence of citric acid in the Sour Skittles. Citric acid is a weak organic acid commonly used as a flavoring agent in sour candies. When the Sour Skittles are dropped into Diet Coke, which contains carbon dioxide gas, the citric acid reacts with the carbon dioxide to produce carbon dioxide bubbles. This reaction creates a fizzy, bubbling effect, causing the Sour Skittles to "explode" or fizz when placed in the Diet Coke.