Find out how hot air balloons work. Material processes - Solids, liquids and gases - Gases Learning Points Air is made up of different gases. The molecules in gas are widely spaced. When heated the molecules of a gas move around and expand in volume. Hot air is less dense than cold air. A Twig Context Film. Reinforce and extend the learning required by the curriculum. Twig’s context films show abstract concepts in action in the real world.