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Solar UFOs

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!


  • Explain the effect of solar energy on a black surface.

  • Demonstrate ways to convert solar energy into thermal energy.


  • Per Demo or Student:
    solar balloon
    sunny, wind-less day (mornings are best)

    Solar balloons may be purchased from Teacher Source or science supply stores (search for Solar Tube or or Solar Bag.)

Key Questions

  • What makes the bag rise up?
  • Have you seen anything hot rise before?
  • Why is the bag black?

What To Do

Do this activity outside in a large, open area. You will need a fairly warm and sunny day for this demonstration.

  1. Tie one end of the solar balloon.
  2. Fill the bag with air and tie the other end.
  3. Attach the string to the bag as you would to a kite


  • Time different sized bags to see if they raise at different rates.
  • Investigate how colour works to explore why black objects heat up.
  • Estimate how much power the heat energy has generated by trying to lift small objects off of the ground.

Other Resources

National Energy Board of Canada About Renewable Energy

BC Hyoro | Conservation

BC Hydro | Sustainability