Due to its high surface area, Mentos allow for a rapid release
of carbon dioxide when dropped into a carbonated liquid. The
escaping bubbles quickly turn into a raging foam, and the pressure
can build dramatically in a restricted container such as a
two-liter bottle. As explained on an episode of MythBusters, diet
soda is most commonly used for the experiment. Diet soda produces a
larger geyser due to the increased carbonation, and the interaction
of the aspartame and other ingredients in the soda. The resulting
geyser (popularly known as a mentos eruption) can shoot as high as
6 meters (20 feet).
http://en.wikipedia.org/wiki/Mentos
