We were at Lancaster University for the Ogden Trust "Cumbria Young Physicist of the Year" Awards. If you're about to start the Lower Sixth, it could be you receiving an award next year! We were shown a trick with liquid nitrogen. Put in an empty water bottle and the liquid nitrogen is so cold that it will cool the air inside dramatically. Any water vapour and carbon dioxide freezes, hence the solid at the end of the bottle. Oxygen condenses at a higher temperature than nitrogen so you do get some liquid oxygen in the bottle. And the nitrogen gas is cooled so that it contracts. With fewer gas atoms inside the bottle and with those that there are inside moving slower, the pressure inside the bottle is reduced. Hence the air pressure outside pushing in compresses the bottle.
