You can do one stage Rocket or a Two Stage Rocket. It took me over 30 tries to get it to get the Two Stage Rocket to work. This was the hardest experiment I have ever done!. I still don’t think NASA will hire me even after my great success. : )
Caution: if it does not launch, wait at least thirty seconds before examining the canister. Usually the cap is not on tight enough and the carbon dioxide has leaked out instead of building up.
The water dissolves the Alka-Seltzer tablet and starts a chemical reaction, which produces carbon dioxide. The carbon dioxide starts to build up, which increases the pressure inside the film canister, until eventually the cap is forced off and the rocket blasts up. This system of thrust is exactly how real rockets work, only they use rocket fuel instead of Alka-Seltzer.
Make sure the film canister lid is tightly fitting or you will only get a disappointing ‘fizz’. You should also clean the canister lip and lid between demonstrations so that no pieces of Alka-Seltzer get stuck between them, ruining the seal.
I used a 1/2 tablet but try a whole one, or even crush the tablet up. The crush tablet will speed up the chemical reaction and it will pop quicker so make sure your pretty fast putting on the lid. : )