Pour two cups of vinegar in the bottom of a stoppered sink or a large bucket. Sprinkle in one cup of baking soda.
Carbon dioxide is produced and remains in the bottom because it is more dense than air.
Light a match and slowly lower it into the sink. You can see the layer of carbon dioxide when the match goes out. Gently blow bubbles with regular bubble solution into the sink or bucket. If the bubbles aren't falling too fast, they will float on the carbon dioxide. Add half a cup of vinegar to a tall glass, or wide mouth jar.