It has been brought to my attention that there are people who have never seen the chemical reaction that results from dropping Mentos candy into a bottle of Diet Coke. You can now buy something called a "Geyser Tube" to release the Mentos from Steve Spangler, who had the first video of the phenomena on his kids' science website/online store. Here is his explanation of what's happening, which (he notes) some scientists think is a physical and not chemical change:
Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of carbon dioxide gas in the soda. In order to form a new bubble, or even to expand a bubble that has already formed, water molecules must push away from each other. It takes extra energy to break this "surface tension." In other words, water "resists" the expansion of bubbles in the soda.
When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy break the surface tension. This disrupts the water mesh, so that it takes less work to expand and form new bubbles. Each Mentos candy has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites - perfect places for carbon dioxide bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy.
Couple this with the fact that the Mentos candies are heavy and sink to the bottom of the bottle and you've got a double-whammy. When all this gas is released, it literally pushes all of the liquid up and out of the bottle in an incredible soda blast. You can see a similar effect when cooking potatoes or pasta are lowered into a pot of boiling water. The water will sometimes boil over because organic materials that leach out of the cooking potatoes or pasta disrupt the tight mesh of water molecules at the surface of the water, making it easier for bubbles and foam to form.
Last time I looked, there were 15 thousand videos on YouTube showing Diet Coke and Mentos, including quite a few purporting to show what happens when you put the candy in your mouth and then take a swig of soda. The one above, of soda geysers choreographed to music, is currently the top-rated -- and my favorite too.