Mentos and Coke
All fizzy drinks such as coke are fizzy because they have carbonated. This means that they have had a lot carbon dioxide (CO2) put in them.
When you drop Mentos in them lots of bubbles of carbon dioxide form and rush out the top because the gas is less dense than the liquid. If you looked at the Mentos very closely then you would see lots of tiny little pits called nucleation points. They are what the bubbles form on.
You could make this in to an experiment instead of a demonstration by changing a variable such as the drink (dirt or original), the amount of Mentos or the amount of drink. You could even see if the temp affects it or if it works with sweets other than Mentos.
The Steve Spangler geyser tube is available from the Steve Spangler science website: http://www.stevespanglerscience.com/store/geyser-tube.html