How do hot air balloons fly?

Hot air is less dense than cold air because its molecules move faster and spread out. When the pilot fires a propane burner, the air inside the envelope heats up, often to around 100 °C, and the balloon becomes buoyant. To descend, the pilot lets the air cool or opens a vent at the top to release hot air. Balloons have no steering, so pilots change altitude to catch winds blowing in different directions. The first manned hot air balloon flight was made by the Montgolfier brothers’ balloon in Paris on 21 November 1783.