Once the peppermint candies are dropped into the Cola bottle, bubbles form on the surface and rise to the top. At the same time, the peppermint candies sink to the bottom of the bottle. These two factors cause the drink to spurt out of the bottle like a geyser. The chemical compounds potassium benzoate and carbon dioxide contained in cola, together with the gum arabic contained in Mentos, lead to excessive foaming.
The geyser occurs in both sugary drinks and diet drinks, although the latter are easier to mop up than the sticky sugared soft drinks. Aspartame reduces surface tension in diet drinks, creating an even higher and more effective geyser.