Materials:
Clear film canisters (these work better than the black and gray ones)
Water
Alka-Seltzer

What To Do:
Fill the film canister half-way with water and add the Alka-Seltzer tablet. Quickly snap the lid back on the canister and stand it upside down. Now stand back!!*

The canister should pop open and fly through the air. Why? The answer lies with the two ingredients in alka-seltzer and similar antacids that help them relieve an upset stomach. Calcium bicarbonate neutralizes acid, while citric acid reacts fizzily to the calcium bicarbonate when combined with water. This reaction produces carbon dioxide (bubbles) and causes the pressure to rise rapidly inside the canister; very quickly, the pressure becomes so high that the canister pops open with great force, and is launched into the air.

More:
Try varying the amount of water in the canisters before launch. Does a nearly-full canister fly farther than one that's only half-full? What effect does adding paper fins or a nose-cone have on the rocket's flight?

* Please do not poke your eye out with an Alka-Seltzer rocket. If you feel you are the kind of person prone to accidents (be honest), maybe you should wear goggles. Or have someone else launch your rockets.