A 2-L bottle of soda (but probably not seltzer: see below)
A package of Mentos brand candies: any flavor. The tube-shaped package works best.
Plenty of room in an outdoor setting
Chill the soda so that not too much CO2 is released just by opening the bottle; make sure it does not get too shaken up
Place the bottle on a flat surface in an open area and tell onlookers to stand back about 10 - 15 ft. (about 3 m for metric sticklers)
Open the bottle of soda and empty the entire package of Mentos into the bottle as quickly as possible
Stand back and watch; there should be an exciting explosion of soda and dissolved candy shooting from the open bottle
Notes About What's Going On
Steve Spangler, director of the National Hands-on Science Institute and the “science guy” on 9NEWS KUSA-TV in Denver originates this demonstration and his web page has plenty of other fun things to do.
On that site he explains that the CO2, which is trapped in solution by water molecules under high pressure, is released more quickly than usual because
The gum arabic in the candy disrupts surface tension, making it easier for the CO2 molecules to escape
As the candy dissolves, pits form on its surface; these pits are great places for bubbles to form
Safety
This demonstration is safe enough for anyone with a bit of common sense to do.
Safety concerns are not high for this demonstration; just try not to get the soda all over yourself!
You should clean up any areas that would be slippery when wet so that you don’t create a hazard.
Cleanup
If appropriate, hose down the affected area.
Some additional notes: Sometimes Mentos can only be bought in a box, not in a roll. You could try putting the candies all into a paper tube so that they are all lined up together. That way they will not bunch up and prevent an even, fast flow into the bottle. Use a standard piece of 8.5 x 11 printing paper.
When I performed this demonstration with seltzer water I got only a half-hearted eruption. Interestingly, I have heard reports of a quite spectacular fountain of soda erupting from the bottle. Invariably, the people telling me about it say that a cola product was used, and not seltzer. Seltzer probably has more CO2 per bottle than cola but lacks any other additives. Perhaps the phosphoric acid, sodium benzoate, cola flavor, caramel color, sugar or aspartame also play a role in whatever makes the soda erupt. I suppose I will have to try it and find out!