What you need:

1.  vinegar
2.  baking soda
3.  a bottle
4.  a balloon

What you do:

 1. Pour the vinegar in the bottle.
 2. Pour the baking soda in the bottle.
 3. Quickly put the balloon on the top of the bottle.
 
 

This experiment shows that the vinegar and baking soda react chemically to push the air left in the bottle to blow up the balloon.