When mixing the two chemicals in baking soda (called sodium bicarbonate) and vinegar (called acetic acid), you cause a chemical reaction to occur. Carbon dioxide is one result of that reaction. Once the carbon dioxide fills up the bottle, it has no where else to go but into the balloon filling it up...
Also on these boards
No helium needed to fill balloons for parties.....just vinegar and baking soda! I need to remember this!