How can air be liquefied?

Question

How can air be liquefied?

Answer 1

In addition to extreme cold, gas can also be liquefied by being placed under extreme pressure.  Helium, in fact, can only be liquefied by a combination of both; it must be brought down to a few specks above absolute zero at about 26 atmospheres to be turned into a liquid.  Higher pressure means a higher temperature can be used and a liquid will still be obtained.

Answer 2

This is really more of a science question but here goes...Nitrogen, which makes up about 78% of our atmosphere liquifies when cooled to -195.8 degrees Celsius.  To liquify a gas you must chill it.  Wkikpedia has a few excellent articles on this subject.