Mr Reimer pours liquid nitrogen over a balloon filled with room temperature gas. The liquid nitrogen is a fluid that boils at -196 oC, so at room temperature you can see liquid nitrogen boiling. The liquid nitrogen is absorbing heat from the container and the air in the room. The liquid nitrogen cools the air in the balloon so the balloon collapses into a flat pancake. When you heat the balloon again the air/gas inside gains kinetic energy and the pressure inside increases and the gas fills the balloon.