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A hot air balloon uses buoyancy to rise in the sky. Since hot air is less dense than cool air, by heating the air inside the balloon, the cooler air outside will push the balloon up. To rise and fall, the pilot needs to control the temperature, and therefore density, of the air inside the balloon. He can do this by adding heat into the balloon, or turning off the heat and allowing the air to cool down. Hotter air will make it rise, while cooler air will make it sink.