How does a balloon fly?
Hot air balloons fly as hot air rises. Hot air rises because it is less dense than the cooler air around it and so it tends to ‘float’ on the cooler air. We use a burner to heat the air in the balloon until it is hotter than the air outside the balloon. Once this is effected the balloon will rise, carrying us with it. If we were to allow the balloon to cool it would begin to descend as it becomes the same temperature as the external air.
If we were to let some of the hot air out of the balloon the same thing would happen because there would no longer be enough hot air in the balloon to lift us. Gas balloons work on a similar principle except that they use helium gas, which is less dense, and therefore lighter than air to achieve the same effect.