Alka Seltzer rocket

You need some old film cannisters and lids along with some alkaseltzer tablets and a bit of plasticine or blu tack. Put a piece of plasticine on the inside of the lid and then break the tablet in half and push it into the plasticine so it cannot come out. Put a small amount of water in the film cannister about 1/4 full. Place the lid on tightly and then invert the whole thing. Wait whilst the water reacts with the tablet and when enough carbon dioxide gas is made the pressure will build up and the rocket will launch. You can investigate different amounts of water and the height that the rocket goes.....