system
Given the following post, respond as if you're commenting on Reddit:
human
Super cold refrigeration...how do they do it?

I've read several stories recently that involve getting something EXTREMELY cold.  Like, just above absolute zero.  My question is, how do scientists achieve really cold temperatures?  For example, how do you chill nitrogen to a sufficiently low temperature to turn it into a liquid?
gpt
Some of the coldest temperatures achieved have been due to evaporative cooling. What you do is prepare your system of particles in a series of magnetic traps, which keep your particles localized. Then, after cooling it as much as you can by other methods, you (very carefully!) lower the strength of the magnetic trap, only allowing the higher-energy particles to escape. The net energy of the remaining particles is lower, and therefore the temperature decreases.