Remember what happens when you drop a Mentos into a 2 liter Diet Coke bottle ? If not, take a look here or here. A device that helps deliver the payload easier - and safer - by the pull of a remote string is available here. But of course, this is an easy remake...
I'm talking about making a Mentos geyser tube, NOT about doing the Mentos + Diet Coke experiment. The geyser tube will allow you to do you experiments remotely, which means cleaner, and safer. So here is what you need in order to make your own Mentos geyser tube:
PVC 1/2" coupling tube (x1) ~ $0.15 @ local home improvement store
PVC 1/2" end cap (x1) ~ $0.30 @ local home improvement store