I don't actually know.
It must be the formula of the two products, that make a chemical reaction inside the liquid.
People like you make me puke. Why post an answer that starts "I don't actually know." If you don't actually know, then shut up and stay away. The reason that Mentos make coke fizz is to do with the surface structure. The surface is covered with tiny pits that release CO2 gas from the drink at a rapid pace, resulting in the froth.