Balloon in liquid nitrogen. An inflated balloon is dipped into a vacuum flask of liquid nitrogen (N2). Liquid nitrogen is a cryogenic fluid, rapidly cooling objects dipped in it. The air inside the balloon is cooled and reduces in volume and pressure. The water vapour in the balloon condenses, followed by the oxygen and nitrogen, causing the balloon to shrink further. Once removed from the liquid nitrogen the contents of the balloon warm up, vaporise and expand, increasing the internal pressure and re-inflating the balloon. This illustrates the relationship between temperature, pressure and volume in gases, the ideal gas law.