Start by making a paper funnel. Do this by rolling a sheet of notebook paper into a tube, leaving the tube just wide enough to allow the Mentos to slide through easily.
Gather all ingredients outside, preferably on a large lawn or in an open field. You'll need a relatively flat surface away from houses, cars, and pets.
Gather your spectators and make sure they stand back. A Coke explosion can sometimes shoot fifteen to twenty feet in the air and what goes up, must come down.
Open the 2 liter bottle of Diet Coke very carefully. You don't want the Coke to lose it's fizz while you're preparing for your experiment. It's the gas inside the bottle that's key to making this experiment work.
Open a package of Mentos candies loosely enough so that the Mentos will easily slide from the wrapper. It's best to leave them back to back as they are in the package to allow for faster and easier insertion into the Diet Coke.
Insert the paper tube into the Diet Coke bottle. Try not to let the tube touch the liquid inside.
Quickly, using the tube, pour the Mentos into the bottle and immediately pull the tube out.
Run! If you're really quick you may be able to do this without getting wet, but don't count on it. Be prepared to do it all over again when your friends see the enormous eruption that explodes from the Coke bottle.