Did you know that Mentos are covered with lots and lots of nucleation sites. Sites where liquids supersaturated with gas like Diet Pepsi and other pops form bubbles to release the gas? The result is a geyser of Diet Pepsi shooting as high as 20 feet and lasting a long time. If you haven't seen a video of some geek shoving Mentos into a 2 liter bottle of Diet Pepsi then you don't get out much. Here is the best in case you missed them.
UPDATE: The "Pepsi Girl" video can only be found here now.