I have always wanted to do this and the boys were game, so last night we tried it. We bought Steve Spanglers geyser tube.
The boys were really excited. You put 7 mentos in the tube and screw the tube to the top of a 2 liter of coke. They say to use diet because it is less sticky. The tube has a little stopper on a string to keep the mentos out of the coke until you want them in.
Why it works (from the package):
Each Mentos chewy candy has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites=perfect places for carbon dioxide bubbles in the soda to form. As soon as the mentos hit the soda, bubbles form all over the surface of the chewy mints and quickly rise to the surface of the liquid. Couple this with the fact that mentos 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."
It was pretty fun!