How do hot air balloons become light enough to leave the ground?

National Geographic (@NatGeo) tweetou às 6:58 PM on sáb, Mar 09, 2013: How do hot air balloons become light enough to leave the ground? http://t.co/M9jlZtEdpt (https://twitter.com/NatGeo/status/310464562682007552) Adquira o aplicativo oficial do Twitter em https://twitter.com/download