After struggling mightily with some slow drains and being unwilling to pay a plumber for so trivial a task, I decided to employ the advice embodied here (Boing Boing: Explosive sink and toilet plunger is a gift from the gods).
I can testify that yes, this is effective. But.
You may as well grab a second set of the CO2 cartridges on your way out. If you have luck like mine, they won’t all be used effectively. First piece of advice, if you’re working with a tub drain, buy a hydraulic/drain bladder (about US$10). You want to use it to plug the overflow completely and in many cases, you may be able to just use that and a lot of plunging. After all, a plunger makes use of the incompressible nature of water: where it fails to work is when there a pocket of compressible air or an outlet that isn’t plugged.
The bladder resembles a thick-skinned rubber balloon, with a hose coupling on one end and a small hole on the other. The principle is that you slip it into the pipe opening, turn on the hose to make it expand and block the pipe, and the water from the other end will eventually push the blockage away. Couple this with the blast of CO2 and you’re bound to get stuff moving. I noticed that as soon as I turned on the water, the overflow filled and the tub drain started backing up. That made it clear that the clog was pretty close to where they join.
Choosing the right side of the plug to use is something you’ll need to think about. I found the one that fits in the drain rather than over it to be more effective.
If you want to use this is a double sink, you’ll need to get someone strong to plug the one you’re not blasting or kneel on the stopper yourself. That was what I ended up doing, and it worked just fine.
But I can say this thing is worth having around. Seven cartridges later, used on two sinks and a bathtub, and things are running smoothly. The cartridges, at US$6 for 4, are not as expensive as a plumbing call. And the addition of the hydraulic bladder to my toolbelt will make old-fashioned plunging a more useful option.
As a side-benefit you can demonstrate the properties of gases to your young learners. Let them feel an intact CO2 cart and a freshly discharged one and marvel at the temperature difference. A bicycle pump would complete the lesson, I suppose.