Jordan thinks that it is a chemical reaction. He thinks that when you mix Diet Coke and Mentos, the gelatin and gum arabic in the candy breaks the surface tension of the Diet Coke which makes it easier for it to create bubbles and after a second or two, too many are formed and it erupts.
 

David thinks that it is a physical reaction. He thinks that when you add Mentos to Diet Coke, the tens of thousands of microscopic holes in the Mentos, give more surface area for the bubbles to hit and then separate. When all of those bubbles are separated, there are too many and it erupts.