Why do Mentos mints foam when you drop them into soda pop?

Mentos have a strange chalky color and texture and they do taste rather like antacid tablets
Ordinarily, water resists the expansion of bubbles in the soda. Water molecules attract each other strongly, and they link together to form a tight mesh around each bubble. It takes energy to push water molecules away from each other to form a new bubble, or to expand a bubble that has already been formed. The phenomenon is called "surface tension".

Now drop a Mentos into the soda. The gelatin and gum arabic from the dissolving candy disrupts the water mesh, so it takes less work to expand bubbles. At the same time, the roughness of the candy surface provides many little nooks and crannies that allow new bubbles to form more quickly (a process called nucleation). As more of the surface dissolves, both processes accelerate, and foam rapidly begins to form.