In this activity, students fill an ultra-thin plastic bag with air, allow the air to heat up under the sun's radiation and watch the bag float. Students observe the transformation of solar energy to thermal energy and are introduced to the concept that hot air rises.
Different colours absorb different amounts of the sun's energy. Black absorbs the most energy of all the colours so it gets hot quickly.
If a black bag is filled with air and put it in the sun, the air in the bag will become hotter than the air around the bag. Hotter air is lighter than cooler air so the "UFO" bag will float because of the sun's energy.
Pro Tip: Be sure to tether your Unidentified Flying Object, lest it float off and cause a UFO sighting!