Mentos-Soda Fountain

If you haven't seen a mentos-soda fountain yet, have a look here:
http://www.youtube.com/watch?v=quYx0uJtzLo
(or simply search youtube for mentos and diet coke*)

And now we know how it works:
http://www.sciam.com/podcast/episode.cfm?id=A0F42D0D-E721-EABE-745EA5C44B66F777

It turns out not to be a chemical reaction at all, but a physical one. In fact, it is nucleation, the same process that I talked about here. This time, instead of bubbles of steam, they are bubbles of carbon dioxide that had been dissolved in the soda. The rough surface of the mentos provides lots of nucleation points at which the bubbles form and the bubbles then form nucleation points for more bubbles, etc. It all adds up to a whole lot of bubbles!! Because of the rapid expansion (bubbles take up a lot more space than liquid), a lot of the bubbly mix gets forced out the small neck of the bottle and voila! A mentos-soda fountain.

*Note that although youtube has mostly mentos and diet coke, it should work with any type of soda. If you are experimenting, it would be worth trying other sweets as well