Have you ever heard of the experiment that calls for mixing Mentos candies with soda? It forms a surprisingly high and dramatic eruption. It's easy to do. First of all, make sure you have the proper ingredients. You'll need a two-liter bottle of Diet Coke and about half a pack of Mentos mint-flavored candies (the kind that come in a long tube-shaped pack like Lifesavers, not the gum or the candies with the liquid filling).
Don't try it with a different flavor or Mentos or a different kind of soda. You might get some kind of a reaction, probably just a bubbling. If you're very lucky you might get a small eruption. But you'll get the best eruption from the combination of mint Mentos and a two-liter bottle of Diet Coke.
All you have to do is place the bottle of Diet Coke on a stable surface that you don't mind having spilled soda all over. A outdoors site, like a driveway or grassy area, is best. Open the bottle. Then open the pack of Mentos and drop about half the pack of mints into the soda, quickly; don't pause between mints. And get out of the way. Quickly!
Here is what it looks like, with my son demonstrating.
http://youtu.be/2whrdAb9SNI