Make your own gas... :) Ok now that you're done laughing, I'm talking about carbon dioxide.
Use a narrow necked jar, vinegar, baking soda and a balloon, Then stand back and watch these molecules and atoms join together to make something new. Carbon Dioxide....a gas you can't otherwise see.
Fill a jar 1/4 of the way with vinegar. Put some baking soda in the balloon. Don't let any soda spill into the jar yet, while you are putting the balloon on the jar. Now quickly lift the balloon to tip all of the soda into the jar. The vinegar will react making bubbles, and filling and blowing up the balloon.
See for yourself.
Happy Experimenting! :)