I raed about this 'fun' experiment in Wikihow. Raynor will sure love this.
Very basic things, simple setup with drastic effect. The eruption is said to be able to reach 18 feet. Be sure not to try this at home or party without consent. (Link) to Wikihow: How to Make a Soda Bottle Volcano
Update: The mad Scientist at EepyBird.com explain how the whole things work. (Link)