When mint or fruit Mentos are dropped into a fresh bottle of Diet Coke, a jet of Coke whooshes out of the bottle's mouth and can reach a height of 10 meters!
Most people assume that this fountain is caused by a chemical reaction, but the truth is that it is a physical phenomenon. Experiments in a 2006 edition of the Discovery Channel program Mythbusters suggested the chemicals responsible for the reaction are gum Arabic and gelatin in the sweets, and caffeine, potassium benzoate and aspartame in the Coke.

To find out more about this, a team of students tested the reactions between Diet Coke and fruit Mentos, mint Mentos, and various ingredients such as other mints, dish-washing detergent, table salt and sand. They also compared reactions using other fizzy liquids such as caffeine-free and sugary colas, as well as soda water and tonic water. The results showed that the fountain height and intensity changes with altering the substance dropped into Coke.

The fountain intensity remained unaffected when caffeine free Coke or Diet Coke were used in the experiment, thereby proving that the phenomenon is not affected by the presence or absence of sugar and caffeine. The results showed that Diet Coke created the most spectacular explosions with either fruit or mint Mentos, the fountain traveling a horizontal distance of up to 7 meters. The experiments also showed that the fountain remained unaffected by the acidity (pH factor) of the substance dropped into the soda.

All this effectively proves that the phenomenon is not a chemical one; rather it is a physical one.
The vigor of the jets depend upon the growth rate of carbon dioxide bubbles. When the Mentos piece was observed under an electron microscope, it was observed to have a rough surface.

The dimply surfaces of Mentos encourage bubble growth, because they efficiently disrupt the polar attractions between water molecules, creating bubble growth sites. So in a rough candy, the ratio of surface area to volume is much more and thus, more bubbles escape from the water every second - forming a geyser like fountain. Dropping in Mentos lowers the surface tension of the liquid. Coatings of Mentos contain gum Arabic, a surfactant that further reduces surface tension in the liquid.

Low surface tension also helps bubbles grow quickly. Measurements showed that the surface tension in water containing the sweetener aspartame is lower than in sugary water, thus explaining why Diet Coke creates more dramatic fountains than sugary Coke.