What you'll need:
water bottle (or glass bottle)
4 TABLESPOON Vinegar
2 TABLESPOONS Baking soda
funnel or piece of paper.
put 4 tablespoons vinegar into the bottle. Open the balloon as wide as you can and using either a funnel or make shift one fill the balloon with baking soda. Cover the lid of the water bottle with the balloon being careful not to spill the baking soda in..yet.
When you're ready dump the contents of the balloon into the water bottle and watch as the baking soda mixes with the vinegar and chemically reacts by making a gas that it then trapped into the balloon giving the illusion that the balloon was blowing itself as if a ghost was doing it ;)
The girls favorite part was when the balloon started to fill with the gas I picked it up and started saying, "oh please don't pop on me...please don't pop!" it did not- but it ended up in teh sink just to be safe!