A flying garbage bag? Now that's a funny thought -- and a memorable way to impress kids that hot air rises!

Shake open a large black plastic garbage bag. "Inflate" the bag by pulling it through the air. (Note the stage direction above; do this activity in a large shady area.) When the bag is fully inflated, twist it closed and tie it with string or twist ties. Tie some string to the bag as a "leash." Then take the inflated bag outside to a spot where it is exposed to the sun and tie it to a pole or a chair. The bag should rest on the ground but have enough string so that it can "fly" while still attached.

Now, you must be patient. Check on the bag periodically as the sun warms it. As the sun warms the bag, it will expand and become lighter than the surrounding air. At first it will struggle to get off the ground. Eventually, though, you will get a rise out of your sun-warmed garbage bag.

BEHIND THE SCENES
Hot-air balloons rise because hot air weighs less than an equal volume of cool air. The key to heating your "balloon" is the black surface of the garbage bag. Dark colors absorb heat rays from the sun. (As a control experiment, you might do the activity with a white plastic trash bag of the same size and weight.) The air inside the black balloon gets warmer and expands. Because the same amount of material takes up more space, the air inside the garbage bag was less dense than the outside air. So the garbage bag was light enough to float.

All you need is a lightweight material to hold the air. And if the bag of air is large, the material doesn't have to be very lightweight. This is because the lifting ability of the hot air goes up with the cube of the diameter of the bag, while the weight of the material goes up only with the square of the diameter.

For small balloons, you can use very thin plastic bags, such as those dry cleaners use to keep the clean clothes from getting dirty on their way home.

For larger balloons, some cheap garbage bags can be used (the cheap ones use a thinner plastic than the expensive ones). Often the thickness of the plastic is mentioned on the package. You want the thinnest material, since it will be the lightest weight. You also want the largest bag you can find. Stores that cater to people who move furniture sell big bags for keeping mattresses clean. Sometimes these bags are too thick and heavy, but you may find some lightweight ones.

To fill the bag with hot air, first seal any holes with cellophane tape. Then seal the open end of the bag with more tape, leaving one small hole at the bottom. This hole should be large enough to allow the nozzle of a hair dryer to fit into it. Fill the bag with hot air from the hair dryer. You may have to be careful not to let the hot air melt the bag.

On a cool day with no wind, the bag should rise into the air and travel quite a distance before it cools and falls. If the bag is large and black, a hot sun will keep the air inside warm enough to provide lift for most of the day. Black bags absorb sunlight and convert it to heat. Clear bags won't do this.

It is not recommended to be using birthday candles or other incendiary materials to keep the bag hot. In the 70s, it was popular to use soda straws in a cross at the open bottom of the dry cleaner bag, and to put birthday candles on the cross to make the bag rise. This caused fires on occasion, although they usually burned out high in the air. The hair dryer is a safer way to fill the bag.

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The first Gordon Bennett balloon race was held in Tuileries, Paris in September 1906. It was won by the American Frank Lahm in his balloon 'United States' with a flight of 420 miles to Fylingdales, Yorkshire, England