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...

Step 1: Gather the Stuff You Need

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:

Materials:

PVC 1/2" coupling tube (x1) ~ $0.15 @ local home improvement store
PVC 1/2" end cap (x1) ~ $0.30 @ local home improvement store
Two litter Coca Cola cap (x1) ~ free
Paper Clip (x1) ~ free
Small cord ~ free

Tools:

Dremel (+ drilling bits + cutter)
Pencil
Loctite Super Glue
Duct Tape

Step 2: Drilling the Cap

Take the 1/2" coupling tube and center it on top of the bottle cap. With a pencil, outline the inside of the coupling tube on the cap. Hold the cap with a pair of plier and you can now machine the cap following the outlines with the Dremel cuter tool. File the chips with the Dremel for a nice finish.

Step 3: Drilling the PVC End Cap

Hold the 1/2" end cap with a pair a pliers and drill a hole on top of the PVC end cap. The guys at Eepybird, in their great tutorial, recommends drilling holes sizes 1/8" or 1/4". You can try drilling one hole only, or more than one hole, or a slot pattern.

Step 4: The Release System

I experimented two different release systems, both located on the 1/2" coupling tube:

The first one is the most classic one: drill two aligned 1/32" holes perpendicular to one end of the 1/2" coupling tube, on each side, where you will then put a paper clip. The setback of this system is that the Coke, in the process, will also escape from these little holes.
The second one works very well too and is more "leak proof": instead of a paper clip, it is using two small magnets (on outside the 1/2" coupling tube and on inside). For this system, instead of drilling small hole, you'll have to machine the coupling tube in order to flatten the tube and thus make a stable base for the magnets.

Whatever system you're using you'll have to attach the paper clip or one of the magnet to a small cord (~ 6ft - 9ft) in order to remotely release the system.

Step 5: Glue All the Pieces Together

Now that all the parts are machined, you can glue them together with the Loctite Super Glue. First the PVC end cap to the PVC coupling tube, and then the bottle cap. Within a few minutes, everything should be sturdy. You can use some duct tape if you like for an even stronger finish.

Step 6: Experiment !

Ok, now your geyser tube is ready to go. Grab some mint Mentos, a 2 litter Diet Coke bottle and your geyser tube. The guys at EepyBird recommends for better and spectacular results in their great tutorial using 5 Mentos per 2 litter bottle and the Diet Coke should be warm (minimum at room temperature, NOT fridge temperature).

Insert 5 mint Mentos into the geyser tube and lock them with the release system (paper clip or magnet)
Place the Diet Coke bottle in a large outside open space (parking lot, etc.)
Open the Diet Coke bottle and carefully screw the loaded geyser tube to it.
Make sure no people or animals are in the line of fire, and trigger the release system with the help of the small cord
Enjoy !