Place a funnel into the top of an empty and dry 2-liter soda bottle.
Pour two cups of white vinegar into the bottle through the funnel. Remove the funnel.
Lay a piece of tissue paper flat onto a table top and pour 1/2 cup of baking soda into the center of the tissue. Fold the edges of the tissue paper together and twist them to create a packet.
Stick the packet through the top of the bottle, then quickly wrap the opening of a balloon over the opening of the bottle.
Place the bottle and balloon onto a flat surface to watch the gas inflate the balloon. When the children observe the balloon inflating, explain how the gas pressure is forcing it's way into the balloon, causing it to inflate, and how the gas inside the balloon is taking up its own space because it has mass.