The Mentos and soda reaction results from the formation of carbon dioxide bubbles on the surface of the Mentos candy. The weight of the candy, which causes it to sink in the soda, also aids the reaction.
The bubbles in soda are the result of carbon dioxide gas added during soda production. Because water molecules hold tightly to one another, they tend to form a protective layer around the carbon dioxide bubbles, making it difficult for bubbles to grow or for new bubbles to form. Dropping Mentos into soda causes the candy to begin to dissolve, breaking up some of the water's surface tension. With the surface tension broken, carbon dioxide bubbles grow and multiply on the candy's surface.
The reaction is further aided by the imperfections on the surface of Mentos. The tiny pits and blemishes form nucleation sites, extra surface where more bubbles can form. As the candy sinks to the bottom, it collects more and more carbon dioxide until the gas escapes the bottle, carrying soda with it in a fountain. This process makes Mentos and soda a physical reaction rather than a chemical reaction.
Adding salt to soda is another way to form carbon dioxide bubbles and cause the soda to foam up. However, the salt does not sink like Mentos and quickly dissolves, so the reaction is less spectacular than the fountain caused by Mentos.