So, you know how if you heat water up past 100 degrees C it turns into water vapour? In exactly the same way, if you heat up nitrogen past – 196 degrees C if turns into nitrogen ‘vapour’ (i.e. the stuff that makes up 70% of the air we breathe). That means if we can find a way to cool it down, below -196C then, hey presto, we have liquid nitrogen.
Nitrogen is pretty good for cooling down cause it turn to liquid at such a cold temperature. Also it’s dead cheap (we’ve got loads of nitrogen in the earth’s atmosphere). Helium is even better, it turns to liquid at a mind bogglingly cold -269 deg C (bearing in mind absolute zero, the lowest temperature you could ever hope to get to is -273 deg C).