Mini-rocket launch
This is a fun experiment demonstrating what happens when acids and bases react. Remember: What goes up must come down - So wear safety specs unless you want a rocket through your eye socket!
Make a tube out of some of the card by wrapping around the film container (you need to be able to get to the lid to snap it closed as quick as possible) Make a cone out of some card and use as the nose of your rocket. Make some rocket fins too
Put 1/2 of an Alka Seltzer tablet into the container and 1/3 fill with warm water
Snap the lid on as fast as you can (or it won't work) and turn upside down
Stand back and watch the rocket launch (don't touch till the reaction has stopped)
Try varying the amount of water, amount of tablet and temperature of the water
(If you want to use vinegar and bicarbonate of soda instead, you'll want to dissolve the bicarbonate of soda in around 1/4 of water and add an equal volume of vinegar to start the reaction)
Like a few of the experiments on this site, this one relies on the reaction between an acid and a base. When an acid and a base react, they produce a salt and a gas. If you carry out the reaction inside a closed container, the gas generated will have no where to go and the pressure will build up till the lid is forced off, or the whole thing explodes. This is pretty much the principle behind most bombs but they use stuff much stronger than simple acid base reactions to generate the gas. This is also why if you hold a firework, like a firecracker, on the palm of your hand you'll get a bit burnt, but if you close your fist around it, your whole hand will be blown off.
Any way, back to the rocket. You may be wondering what acids and bases have to do with Alka Seltzer - well, the tablets are in fact made from sodium bicarbonate and dehydrated citric acid, this makes them fizz when you drop them in water. This is why you can replace the tablet with vinegar (or lemon juice) and baking powder in the experiment. The reaction produces CO2 gas and it is this than launches the rocket. The reaction is faster if you use warm water and this will make the rocket go higher.