Need a science fair project? Here is one that is fun, easy to set up and entertaining to observers, especially younger ones as it can be performed as somewhat of a “magic” trick. This also then presents the opportunity to discuss the idea of magic being often more of a science trick.
To set up what I like to call “The Balancing Brown Bags of Science” you will need two brown paper lunch bags, string, a meter stick, a ring stand, a large pitcher, vinegar and baking soda. First, using the string, hang a paper bag from each end of the meter stick and then hang the meter stick from the ring stand by tying the string around the exact center of the stick. This is the tricky part because at this point you want the stick hanging from the stand, parallel to the floor, with a paper bag hanging from each end. The difficulty lies in finding the perfect balance between the stick, bags and string so that this all becomes somewhat of a balance. Pieces of tape placed appropriately can help to find the perfect balance. At this point you will have a delicate balance displaying “balanced room air” filling the two empty bags. Now in the pitcher dump some baking soda, maybe 1/4 cup, followed by 1/2 to 3/4 cup of vinegar and swirl around so that is mixes (these amounts can be adjusted to desired effect). The mixing of vinegar and baking soda creates carbon dioxide which is much more dense than normal surrounding air and will flow out of the pitcher and towards the ground much like a liquid. So after mixing, tip the pitcher over one of the bags, not enough for the mixture to flow out but enough for the carbon dioxide gas to flow out, fill the bag and “magically” tip the scale without touching it. The dense gas will eventually dissipate and the balance will return to normal, or you can blow air into the bag to remove it. Another option is to very carefully light a candle beneath one of the bags to warm the air, lower its density and cause it to rise.
Having a sufficient amount of air flow should be considered since this creates carbon dioxide gas if a large number of smaller children will be in attendance since this dense gas settles low.