The Science of the film canister rocket
When the alka-seltzer is put into the water in the film canister, it starts to "fizz" creating a lot of carbon dioxide gas. When the lid is put on, it traps the gas inside and so the pressure increases a lot.
Eventually the pressure increases enough to "pop" the lid off and you have a "rocket"!
If you are going to try this yourself, you will find it even more dramatic if you put the canister upside-down (i.e. on the lid). However, be careful to stand well back as it can hurt or could damage your eyes.