Tip: If you've just moved into a new neighborhood, walk outside and announce to any nearby children that you are going to make a two-liter bottle of soda explode. They will immediately grab their friends, and the parents of all the kids will come out to find out what is going on and you will meet lots of people. The kids in the neighborhood will always talk about what fabulous parents you are. They will tell your kids they wish their parents were so cool.
Basically, you drop a tube of Mentos candy into a two-liter bottle of diet coke. You have to do it fast, as they all have to drop in at once. You can't drop them in one at a time. That's why we bought the geyser tube.
If you want to know the science behind why the soda explodes in a geyser, check it out here.