Got to make you smile first thing on a Monday morning :)
What happens when you combine 200 liters of Diet Coke and over 500 Mentos mints? It's amazing and completely insane.The first part of this video demonstrates a simple geyser, and the second part shows just how extreme it can get. Over one hundred jets of soda fly into the air in less than three minutes.It's a hysterical and spectacular mint-powered version of the Bellagio Fountains in Las Vegas, brought to you by the mad scientists at EepyBird.com.
and not to spoil the fun but in case you are wondering; copied from slashdot is the answer.
Not chemical, so it should work just as well with other sodas/nucleation devices:
"These chemists are saying that the primary cause is physical, not chemical. Their explanation: nucleation sites. If you have a liquid that is supersaturated with gas (like soda, which is pumped full of carbon dioxide), a nucleation site is a place where the gas is able to form bubbles. Nucleation sites can be scratches on a surface or specks of dust - anywhere that you have a high surface area in a very small volume. That's where bubbles can form.
Mentos seem to be loaded with nucleation sites. In other words, there are so many microscopic nooks and crannies on the surface of a Mento that an incredible number of bubbles will form when you drop it in a bottle of soda. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically. Before you know it, you've got a big geyser happening!"