Adding Mentos to a bottle of soda causes the carbon dioxide contained within the beverage to react. This reaction causes the soda to erupt and spray out of the container.
Once the Mentos are placed in the soda, carbon dioxide bubbles begin to form on the surface of the mints. Mentos candies are covered by small holes which increase their total surface area, potentiating the reaction. Mentos are also more dense than the soda, causing them to sink as the gas bubbles rise to the surface. The combination of these two factors allows the carbon dioxide bubbles to form very rapidly until they force the soda out off the bottle in a jet-like geyser. The same effect can be achieved by using rock salt in place of Mentos.