Budding chemists aren't the only ones who will get a kick out of this enjoyable experiment that teaches something about how the natural world works. When baking soda and vinegar come into contact, they form carbon dioxide. This gas fills the bottle and can't escape, so it rushes into the balloon, causing it to inflate.
Kids Good Things 2006, Volume 2006 Special Issue 2006
Fill a balloon with air without blowing into it: Pour 4 tablespoons of vinegar into a clean bottle.
Use a funnel to fill the balloon with 1 tablespoon of baking soda. Be sure that the bulb of the balloon falls to one side as you carefully slip its mouth over the neck of the bottle.
Lift the bulb to let the baking soda fall into the bottle. The balloon will inflate all by itself!