I've seen this experiment several places, so I take no responsibility it. We've also done this experiment a few times before and each time it gets my boys' attention. Most recently, I've seen this in the book Child's Play by Leslie Hamilton, listed as Magic Balloon.
You'll need a balloon, small pop bottle, baking soda, vinegar and a funnel.
Use the funnel to put about 1 tablespoon of baking soda into the balloon. Use it again to put 1 to 2 inches of vinegar into the pop bottle.
Secure the mouth of the balloon to the mouth of the pop bottle by making sure the baking soda is the the bottom of the balloon and then folding the balloon over so the baking soda doesn't fall into the balloon. Work with only the mouth of the balloon. Yes, I know, this takes some coordination!
Hold onto the balloon tightly while your child lifts the baking soda-filled part of the balloon, letting the baking soda fall into the vinegar.
What happens? Gas is formed by mixing the baking soda (a base) with the vinegar (an acid) and the gas fills the balloon!
Can you tell there are two underlying themes this week? If you guessed 'what we did this summer' and 'balloons' then you are right!