The bubbles that are created are filled with carbon dioxide. The carbon dioxide is a gas that forms when the vinegar (an acid) reacts with the baking soda (a base). For all you bakers out there, this is also what makes cakes and quick breads (the no yeast kind) get all nice and fluffy.
You can make your volcano more realistic by covering the foil layer with papier-mâché to look like a rocky volcanic mountain. Also, try adding a few drops of red food coloring to the vinegar and water mixture for red-hot lava.
To make your volcano: Attach the paper cup to the plate by taping the bottom of the cup to the middle of the plate.
Tear off a piece of foil large enough to completely cover the cup and plate.
Place the foil over the cup and plate and turn the foil under the plate edge.
Tape the foil in place.
Poke a hole through the foil into the middle of the cup.
Use scissors to make slits from the middle of the cup to the inside edge of the cup. Tape the foil to the inside of the cup.
To make your volcano erupt: Place the volcano on a pan or tray (or you'll get lava all over the place!)
Fill the volcano with 2 tablespoons of water and stir in a tablespoon of baking soda until it dissolves.
Measure 2 tablespoons of vinegar into a separate cup.
Pour the vinegar, all at once into the water/baking soda mixture and watch your lava bubble up!